Hello!
This series contains documentation updates. Some of these describe changes
that are carried out in another branch, but are kept separate to allow
reviewers to easily find the intent of those changes.
1. Add design documentation on interruption of NMI handlers.
2. Fix broken RCU-requirements link to LKML archive, courtesy of
Joel Fernandes.
3. Improve rcu_dynticks::dynticks documentation, courtesy of
Joel Fernandes.
4. Update documentation for removal of RCU-bh update machinery.
5. Fix broken HTML directive.
6. Update documentation for removal of RCU-sched update machinery.
Thanx, Paul
------------------------------------------------------------------------
RCU/Design/Data-Structures/Data-Structures.html | 8
RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html | 9
RCU/Design/Requirements/Requirements.html | 164 ++--------
RCU/stallwarn.txt | 13
RCU/whatisRCU.txt | 3
admin-guide/kernel-parameters.txt | 16
kernel-per-CPU-kthreads.txt | 2
7 files changed, 73 insertions(+), 142 deletions(-)
Make Requirements.html talk about how NMI handlers can take what appear
to RCU to be normal interrupts.
Signed-off-by: Paul E. McKenney <[email protected]>
---
.../RCU/Design/Requirements/Requirements.html | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 49690228b1c6..089a8e8faac1 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2275,6 +2275,17 @@ he also kindly surprised me with
<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
that meets this requirement.
+<p>
+Furthermore, NMI handlers can be interrupted by what appear to RCU
+to be normal interrupts.
+One way that this can happen is for code that directly invokes
+<tt>rcu_irq_enter()</tt> and </tt>rcu_irq_exit()</tt> to be called
+from an NMI handler.
+This astonishing fact of life prompted the current code structure,
+which has <tt>rcu_irq_enter()</tt> invoking <tt>rcu_nmi_enter()</tt>
+and <tt>rcu_irq_exit()</tt> invoking <tt>rcu_nmi_exit()</tt>.
+And yes, I also learned of this requirement the hard way.
+
<h3><a name="Loadable Modules">Loadable Modules</a></h3>
<p>
--
2.17.1
Although RCU-sched persists in !PREEMPT kernels, in the PREEMPT case its
update API is now defined in terms of that of RCU-preempt, so this commit
updates the documentation accordingly. While in the area, this commit
removes the documentation for the now-obsolete synchronize_rcu_mult()
and clarifies the Tasks RCU documentation.
Signed-off-by: Paul E. McKenney <[email protected]>
---
.../RCU/Design/Requirements/Requirements.html | 133 +++---------------
1 file changed, 21 insertions(+), 112 deletions(-)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 68bedbe8da76..701b5c53607f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1306,8 +1306,6 @@ doing so would degrade real-time response.
<p>
This non-requirement appeared with preemptible RCU.
-If you need a grace period that waits on non-preemptible code regions, use
-<a href="#Sched Flavor">RCU-sched</a>.
<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
@@ -2165,14 +2163,9 @@ however, this is not a panacea because there would be severe restrictions
on what operations those callbacks could invoke.
<p>
-Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
-<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
-(<a href="#Bottom-Half Flavor">discussed below</a>),
-<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>,
+Perhaps surprisingly, <tt>synchronize_rcu()</tt> and
<tt>synchronize_rcu_expedited()</tt>,
-<tt>synchronize_rcu_bh_expedited()</tt>, and
-<tt>synchronize_sched_expedited()</tt>
-will all operate normally
+will operate normally
during very early boot, the reason being that there is only one CPU
and preemption is disabled.
This means that the call <tt>synchronize_rcu()</tt> (or friends)
@@ -2862,11 +2855,9 @@ described in a separate section.
<ol>
<li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a>
-<li> <a href="#Sched Flavor">Sched Flavor</a>
+<li> <a href="#Sched Flavor">Sched Flavor (Historical)</a>
<li> <a href="#Sleepable RCU">Sleepable RCU</a>
<li> <a href="#Tasks RCU">Tasks RCU</a>
-<li> <a href="#Waiting for Multiple Grace Periods">
- Waiting for Multiple Grace Periods</a>
</ol>
<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a></h3>
@@ -2942,7 +2933,16 @@ However, the update-side APIs are now simple wrappers for other RCU
flavors, namely RCU-sched in CONFIG_PREEMPT=n kernels and RCU-preempt
otherwise.
-<h3><a name="Sched Flavor">Sched Flavor</a></h3>
+<h3><a name="Sched Flavor">Sched Flavor (Historical)</a></h3>
+
+<p>
+The RCU-sched flavor of RCU has since been expressed in terms of
+the other RCU flavors as part of a consolidation of the three
+flavors into a single flavor.
+The read-side API remains, and continues to disable preemption and to
+be accounted for by lockdep.
+Much of the material in this section is therefore strictly historical
+in nature.
<p>
Before preemptible RCU, waiting for an RCU grace period had the
@@ -3162,94 +3162,14 @@ The tasks-RCU API is quite compact, consisting only of
<tt>call_rcu_tasks()</tt>,
<tt>synchronize_rcu_tasks()</tt>, and
<tt>rcu_barrier_tasks()</tt>.
-
-<h3><a name="Waiting for Multiple Grace Periods">
-Waiting for Multiple Grace Periods</a></h3>
-
-<p>
-Perhaps you have an RCU protected data structure that is accessed from
-RCU read-side critical sections, from softirq handlers, and from
-hardware interrupt handlers.
-That is three flavors of RCU, the normal flavor, the bottom-half flavor,
-and the sched flavor.
-How to wait for a compound grace period?
-
-<p>
-The best approach is usually to “just say no!” and
-insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
-around each RCU read-side critical section, regardless of what
-environment it happens to be in.
-But suppose that some of the RCU read-side critical sections are
-on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
-is not a viable option, so that <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> are not free.
-What then?
-
-<p>
-You <i>could</i> wait on all three grace periods in succession, as follows:
-
-<blockquote>
-<pre>
- 1 synchronize_rcu();
- 2 synchronize_rcu_bh();
- 3 synchronize_sched();
-</pre>
-</blockquote>
-
-<p>
-This works, but triples the update-side latency penalty.
-In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
-may be used to wait on all three flavors of grace period concurrently:
-
-<blockquote>
-<pre>
- 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
-</pre>
-</blockquote>
-
-<p>
-But what if it is necessary to also wait on SRCU?
-This can be done as follows:
-
-<blockquote>
-<pre>
- 1 static void call_my_srcu(struct rcu_head *head,
- 2 void (*func)(struct rcu_head *head))
- 3 {
- 4 call_srcu(&my_srcu, head, func);
- 5 }
- 6
- 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
-</pre>
-</blockquote>
-
-<p>
-If you needed to wait on multiple different flavors of SRCU
-(but why???), you would need to create a wrapper function resembling
-<tt>call_my_srcu()</tt> for each SRCU flavor.
-
-<table>
-<tr><th> </th></tr>
-<tr><th align="left">Quick Quiz:</th></tr>
-<tr><td>
- But what if I need to wait for multiple RCU flavors, but I also need
- the grace periods to be expedited?
-</td></tr>
-<tr><th align="left">Answer:</th></tr>
-<tr><td bgcolor="#ffffff"><font color="ffffff">
- If you are using expedited grace periods, there should be less penalty
- for waiting on them in succession.
- But if that is nevertheless a problem, you can use workqueues
- or multiple kthreads to wait on the various expedited grace
- periods concurrently.
-</font></td></tr>
-<tr><td> </td></tr>
-</table>
-
-<p>
-Again, it is usually better to adjust the RCU read-side critical sections
-to use a single flavor of RCU, but when this is not feasible, you can use
-<tt>synchronize_rcu_mult()</tt>.
+In <tt>CONFIG_PREEMPT=n</tt> kernels, trampolines cannot be preempted,
+so these APIs map to
+<tt>call_rcu()</tt>,
+<tt>synchronize_rcu()</tt>, and
+<tt>rcu_barrier()</tt>, respectively.
+In <tt>CONFIG_PREEMPT=y</tt> kernels, trampolines can be preempted,
+and these three APIs are therefore implemented by separate functions
+that check for voluntary context switches.
<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
@@ -3260,12 +3180,6 @@ If this becomes a serious problem, it will be necessary to rework the
grace-period state machine so as to avoid the need for the additional
latency.
-<p>
-Expedited grace periods scan the CPUs, so their latency and overhead
-increases with increasing numbers of CPUs.
-If this becomes a serious problem on large systems, it will be necessary
-to do some redesign to avoid this scalability problem.
-
<p>
RCU disables CPU hotplug in a few places, perhaps most notably in the
<tt>rcu_barrier()</tt> operations.
@@ -3310,11 +3224,6 @@ Please note that arrangements that require RCU to remap CPU numbers will
require extremely good demonstration of need and full exploration of
alternatives.
-<p>
-There is an embarrassingly large number of flavors of RCU, and this
-number has been increasing over time.
-Perhaps it will be possible to combine some at some future date.
-
<p>
RCU's various kthreads are reasonably recent additions.
It is quite likely that adjustments will be required to more gracefully
--
2.17.1
This commit adds the needed "<".
Signed-off-by: Paul E. McKenney <[email protected]>
---
.../Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
index ffd612bfa436..e62c7c34a369 100644
--- a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
+++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
@@ -157,7 +157,7 @@ whether or not the current CPU is in an RCU read-side critical section.
The best that <tt>sync_sched_exp_handler()</tt> can do is to check
for idle, on the off-chance that the CPU went idle while the IPI
was in flight.
-If the CPU is idle, then tt>sync_sched_exp_handler()</tt> reports
+If the CPU is idle, then <tt>sync_sched_exp_handler()</tt> reports
the quiescent state.
<p>
--
2.17.1
The RCU-bh update API is now defined in terms of that of RCU-bh and
RCU-sched, so this commit updates the documentation accordingly.
Signed-off-by: Paul E. McKenney <[email protected]>
---
.../Expedited-Grace-Periods.html | 7 +++----
.../RCU/Design/Requirements/Requirements.html | 16 ++++++++++++++--
Documentation/RCU/stallwarn.txt | 13 +++++--------
Documentation/RCU/whatisRCU.txt | 3 ++-
Documentation/admin-guide/kernel-parameters.txt | 16 ++++++++--------
Documentation/kernel-per-CPU-kthreads.txt | 2 +-
6 files changed, 33 insertions(+), 24 deletions(-)
diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
index 7394f034be65..ffd612bfa436 100644
--- a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
+++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
@@ -12,10 +12,9 @@ high efficiency and minimal disturbance, expedited grace periods accept
lower efficiency and significant disturbance to attain shorter latencies.
<p>
-There are three flavors of RCU (RCU-bh, RCU-preempt, and RCU-sched),
-but only two flavors of expedited grace periods because the RCU-bh
-expedited grace period maps onto the RCU-sched expedited grace period.
-Each of the remaining two implementations is covered in its own section.
+There are two flavors of RCU (RCU-preempt and RCU-sched), with an earlier
+third RCU-bh flavor having been implemented in terms of the other two.
+Each of the two implementations is covered in its own section.
<ol>
<li> <a href="#Expedited Grace Period Design">
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 51f39f65002d..68bedbe8da76 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2861,7 +2861,7 @@ The other four flavors are listed below, with requirements for each
described in a separate section.
<ol>
-<li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
+<li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a>
<li> <a href="#Sched Flavor">Sched Flavor</a>
<li> <a href="#Sleepable RCU">Sleepable RCU</a>
<li> <a href="#Tasks RCU">Tasks RCU</a>
@@ -2869,7 +2869,16 @@ described in a separate section.
Waiting for Multiple Grace Periods</a>
</ol>
-<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
+<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a></h3>
+
+<p>
+The RCU-bh flavor of RCU has since been expressed in terms of
+the other RCU flavors as part of a consolidation of the three
+flavors into a single flavor.
+The read-side API remains, and continues to disable softirq and to
+be accounted for by lockdep.
+Much of the material in this section is therefore strictly historical
+in nature.
<p>
The softirq-disable (AKA “bottom-half”,
@@ -2929,6 +2938,9 @@ includes
<tt>call_rcu_bh()</tt>,
<tt>rcu_barrier_bh()</tt>, and
<tt>rcu_read_lock_bh_held()</tt>.
+However, the update-side APIs are now simple wrappers for other RCU
+flavors, namely RCU-sched in CONFIG_PREEMPT=n kernels and RCU-preempt
+otherwise.
<h3><a name="Sched Flavor">Sched Flavor</a></h3>
diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index f99cf11b314b..491043fd976f 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -16,12 +16,9 @@ o A CPU looping in an RCU read-side critical section.
o A CPU looping with interrupts disabled.
-o A CPU looping with preemption disabled. This condition can
- result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
- stalls.
+o A CPU looping with preemption disabled.
-o A CPU looping with bottom halves disabled. This condition can
- result in RCU-sched and RCU-bh stalls.
+o A CPU looping with bottom halves disabled.
o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
without invoking schedule(). If the looping in the kernel is
@@ -87,9 +84,9 @@ o A hardware failure. This is quite unlikely, but has occurred
This resulted in a series of RCU CPU stall warnings, eventually
leading the realization that the CPU had failed.
-The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
-warning. Note that SRCU does -not- have CPU stall warnings. Please note
-that RCU only detects CPU stalls when there is a grace period in progress.
+The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning.
+Note that SRCU does -not- have CPU stall warnings. Please note that
+RCU only detects CPU stalls when there is a grace period in progress.
No grace period, no CPU stall warnings.
To diagnose the cause of the stall, inspect the stack traces.
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index c2a7facf7ff9..86d82f7f3500 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -934,7 +934,8 @@ c. Do you need to treat NMI handlers, hardirq handlers,
d. Do you need RCU grace periods to complete even in the face
of softirq monopolization of one or more of the CPUs? For
example, is your code subject to network-based denial-of-service
- attacks? If so, you need RCU-bh.
+ attacks? If so, you should disable softirq across your readers,
+ for example, by using rcu_read_lock_bh().
e. Is your workload too update-intensive for normal use of
RCU, but inappropriate for other synchronization mechanisms?
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9871e649ffef..9e69ec54b7d3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3534,14 +3534,14 @@
In kernels built with CONFIG_RCU_NOCB_CPU=y, set
the specified list of CPUs to be no-callback CPUs.
- Invocation of these CPUs' RCU callbacks will
- be offloaded to "rcuox/N" kthreads created for
- that purpose, where "x" is "b" for RCU-bh, "p"
- for RCU-preempt, and "s" for RCU-sched, and "N"
- is the CPU number. This reduces OS jitter on the
- offloaded CPUs, which can be useful for HPC and
- real-time workloads. It can also improve energy
- efficiency for asymmetric multiprocessors.
+ Invocation of these CPUs' RCU callbacks will be
+ offloaded to "rcuox/N" kthreads created for that
+ purpose, where "x" is "p" for RCU-preempt, and
+ "s" for RCU-sched, and "N" is the CPU number.
+ This reduces OS jitter on the offloaded CPUs,
+ which can be useful for HPC and real-time
+ workloads. It can also improve energy efficiency
+ for asymmetric multiprocessors.
rcu_nocb_poll [KNL]
Rather than requiring that offloaded CPUs
diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index 0f00f9c164ac..23b0c8b20cd1 100644
--- a/Documentation/kernel-per-CPU-kthreads.txt
+++ b/Documentation/kernel-per-CPU-kthreads.txt
@@ -321,7 +321,7 @@ To reduce its OS jitter, do at least one of the following:
to do.
Name:
- rcuob/%d, rcuop/%d, and rcuos/%d
+ rcuop/%d and rcuos/%d
Purpose:
Offload RCU callbacks from the corresponding CPU.
--
2.17.1
From: "Joel Fernandes (Google)" <[email protected]>
The very useful RCU Data-Structures describes that the dynticks counter
of the rcu_dynticks data structure is incremented when we transitions to
or from dynticks-idle mode. However it doesn't mention that it is also
incremented due to transitions to and from user mode which for dynticks
purposes is an extended quiescent state.
I found this with tracing calls to rcu_dynticks_eqs_enter which can also
happen from rcu_user_enter. Lets add this information to the
Data-Structures document.
Signed-off-by: Joel Fernandes (Google) <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
---
.../RCU/Design/Data-Structures/Data-Structures.html | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index f5120a00f511..50be87e59937 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -1227,9 +1227,11 @@ to overflow the counter, this approach corrects the
CPU enters the idle loop from process context.
</p><p>The <tt>->dynticks</tt> field counts the corresponding
-CPU's transitions to and from dyntick-idle mode, so that this counter
-has an even value when the CPU is in dyntick-idle mode and an odd
-value otherwise.
+CPU's transitions to and from either dyntick-idle or user mode, so
+that this counter has an even value when the CPU is in dyntick-idle
+mode or user mode and an odd value otherwise. The transitions to/from
+user mode need to be counted for user mode adaptive-ticks support
+(see timers/NO_HZ.txt).
</p><p>The <tt>->rcu_need_heavy_qs</tt> field is used
to record the fact that the RCU core code would really like to
--
2.17.1
From: "Joel Fernandes (Google)" <[email protected]>
Two of the Requirements.html LKML links are broken. This patch changes
them to use the archive from lore.kernel.org, which works fine.
Signed-off-by: Joel Fernandes (Google) <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
---
Documentation/RCU/Design/Requirements/Requirements.html | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 089a8e8faac1..51f39f65002d 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2269,10 +2269,10 @@ Thankfully, RCU update-side primitives, including
The name notwithstanding, some Linux-kernel architectures
can have nested NMIs, which RCU must handle correctly.
Andy Lutomirski
-<a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a>
+<a href="https://lkml.kernel.org/r/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a>
with this requirement;
he also kindly surprised me with
-<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
+<a href="https://lkml.kernel.org/r/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
that meets this requirement.
<p>
--
2.17.1
On Wed, Aug 29, 2018 at 02:17:20PM -0700, Paul E. McKenney wrote:
> The RCU-bh update API is now defined in terms of that of RCU-bh and
> RCU-sched, so this commit updates the documentation accordingly.
>
> Signed-off-by: Paul E. McKenney <[email protected]>
And an update consolidating this patch with 6/6 and with the documentation
pieces of 4ca0508f7870 ("rcu: Remove rsp parameter from rcu_node tree
accessor macros").
Thanx, Paul
------------------------------------------------------------------------
commit 77095901b895a64b6d775879b54c73472ba21e68
Author: Paul E. McKenney <[email protected]>
Date: Mon Jul 2 08:25:57 2018 -0700
doc: Update removal of RCU-bh/sched update machinery
The RCU-bh update API is now defined in terms of that of RCU-bh and
RCU-sched, so this commit updates the documentation accordingly.
In addition, although RCU-sched persists in !PREEMPT kernels, in
the PREEMPT case its update API is now defined in terms of that of
RCU-preempt, so this commit also updates the documentation accordingly.
While in the area, this commit removes the documentation for the
now-obsolete synchronize_rcu_mult() and clarifies the Tasks RCU
documentation.
Signed-off-by: Paul E. McKenney <[email protected]>
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index 50be87e59937..1d2051c0c3fc 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -1374,8 +1374,7 @@ that is, if the CPU is currently idle.
Accessor Functions</a></h3>
<p>The following listing shows the
-<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
-<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
+<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt> and
<tt>rcu_for_each_leaf_node()</tt> function and macros:
<pre>
@@ -1388,13 +1387,9 @@ Accessor Functions</a></h3>
7 for ((rnp) = &(rsp)->node[0]; \
8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
9
- 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
- 11 for ((rnp) = &(rsp)->node[0]; \
- 12 (rnp) < (rsp)->level[NUM_RCU_LVLS - 1]; (rnp)++)
- 13
- 14 #define rcu_for_each_leaf_node(rsp, rnp) \
- 15 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
- 16 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+ 10 #define rcu_for_each_leaf_node(rsp, rnp) \
+ 11 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
+ 12 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
</pre>
<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
@@ -1407,10 +1402,7 @@ macro takes advantage of the layout of the <tt>rcu_node</tt>
structures in the <tt>rcu_state</tt> structure's
<tt>->node[]</tt> array, performing a breadth-first traversal by
simply traversing the array in order.
-The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
-similarly, but traverses only the first part of the array, thus excluding
-the leaf <tt>rcu_node</tt> structures.
-Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
+Similarly, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
the last part of the array, thus traversing only the leaf
<tt>rcu_node</tt> structures.
@@ -1418,15 +1410,14 @@ the last part of the array, thus traversing only the leaf
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
- What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
+ What does
<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
contains only a single node?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In the single-node case,
- <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
- and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
+ <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
</font></td></tr>
<tr><td> </td></tr>
</table>
diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
index 7394f034be65..ffd612bfa436 100644
--- a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
+++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
@@ -12,10 +12,9 @@ high efficiency and minimal disturbance, expedited grace periods accept
lower efficiency and significant disturbance to attain shorter latencies.
<p>
-There are three flavors of RCU (RCU-bh, RCU-preempt, and RCU-sched),
-but only two flavors of expedited grace periods because the RCU-bh
-expedited grace period maps onto the RCU-sched expedited grace period.
-Each of the remaining two implementations is covered in its own section.
+There are two flavors of RCU (RCU-preempt and RCU-sched), with an earlier
+third RCU-bh flavor having been implemented in terms of the other two.
+Each of the two implementations is covered in its own section.
<ol>
<li> <a href="#Expedited Grace Period Design">
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 51f39f65002d..701b5c53607f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1306,8 +1306,6 @@ doing so would degrade real-time response.
<p>
This non-requirement appeared with preemptible RCU.
-If you need a grace period that waits on non-preemptible code regions, use
-<a href="#Sched Flavor">RCU-sched</a>.
<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
@@ -2165,14 +2163,9 @@ however, this is not a panacea because there would be severe restrictions
on what operations those callbacks could invoke.
<p>
-Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
-<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
-(<a href="#Bottom-Half Flavor">discussed below</a>),
-<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>,
+Perhaps surprisingly, <tt>synchronize_rcu()</tt> and
<tt>synchronize_rcu_expedited()</tt>,
-<tt>synchronize_rcu_bh_expedited()</tt>, and
-<tt>synchronize_sched_expedited()</tt>
-will all operate normally
+will operate normally
during very early boot, the reason being that there is only one CPU
and preemption is disabled.
This means that the call <tt>synchronize_rcu()</tt> (or friends)
@@ -2861,15 +2854,22 @@ The other four flavors are listed below, with requirements for each
described in a separate section.
<ol>
-<li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
-<li> <a href="#Sched Flavor">Sched Flavor</a>
+<li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a>
+<li> <a href="#Sched Flavor">Sched Flavor (Historical)</a>
<li> <a href="#Sleepable RCU">Sleepable RCU</a>
<li> <a href="#Tasks RCU">Tasks RCU</a>
-<li> <a href="#Waiting for Multiple Grace Periods">
- Waiting for Multiple Grace Periods</a>
</ol>
-<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
+<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a></h3>
+
+<p>
+The RCU-bh flavor of RCU has since been expressed in terms of
+the other RCU flavors as part of a consolidation of the three
+flavors into a single flavor.
+The read-side API remains, and continues to disable softirq and to
+be accounted for by lockdep.
+Much of the material in this section is therefore strictly historical
+in nature.
<p>
The softirq-disable (AKA “bottom-half”,
@@ -2929,8 +2929,20 @@ includes
<tt>call_rcu_bh()</tt>,
<tt>rcu_barrier_bh()</tt>, and
<tt>rcu_read_lock_bh_held()</tt>.
+However, the update-side APIs are now simple wrappers for other RCU
+flavors, namely RCU-sched in CONFIG_PREEMPT=n kernels and RCU-preempt
+otherwise.
-<h3><a name="Sched Flavor">Sched Flavor</a></h3>
+<h3><a name="Sched Flavor">Sched Flavor (Historical)</a></h3>
+
+<p>
+The RCU-sched flavor of RCU has since been expressed in terms of
+the other RCU flavors as part of a consolidation of the three
+flavors into a single flavor.
+The read-side API remains, and continues to disable preemption and to
+be accounted for by lockdep.
+Much of the material in this section is therefore strictly historical
+in nature.
<p>
Before preemptible RCU, waiting for an RCU grace period had the
@@ -3150,94 +3162,14 @@ The tasks-RCU API is quite compact, consisting only of
<tt>call_rcu_tasks()</tt>,
<tt>synchronize_rcu_tasks()</tt>, and
<tt>rcu_barrier_tasks()</tt>.
-
-<h3><a name="Waiting for Multiple Grace Periods">
-Waiting for Multiple Grace Periods</a></h3>
-
-<p>
-Perhaps you have an RCU protected data structure that is accessed from
-RCU read-side critical sections, from softirq handlers, and from
-hardware interrupt handlers.
-That is three flavors of RCU, the normal flavor, the bottom-half flavor,
-and the sched flavor.
-How to wait for a compound grace period?
-
-<p>
-The best approach is usually to “just say no!” and
-insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
-around each RCU read-side critical section, regardless of what
-environment it happens to be in.
-But suppose that some of the RCU read-side critical sections are
-on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
-is not a viable option, so that <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> are not free.
-What then?
-
-<p>
-You <i>could</i> wait on all three grace periods in succession, as follows:
-
-<blockquote>
-<pre>
- 1 synchronize_rcu();
- 2 synchronize_rcu_bh();
- 3 synchronize_sched();
-</pre>
-</blockquote>
-
-<p>
-This works, but triples the update-side latency penalty.
-In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
-may be used to wait on all three flavors of grace period concurrently:
-
-<blockquote>
-<pre>
- 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
-</pre>
-</blockquote>
-
-<p>
-But what if it is necessary to also wait on SRCU?
-This can be done as follows:
-
-<blockquote>
-<pre>
- 1 static void call_my_srcu(struct rcu_head *head,
- 2 void (*func)(struct rcu_head *head))
- 3 {
- 4 call_srcu(&my_srcu, head, func);
- 5 }
- 6
- 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
-</pre>
-</blockquote>
-
-<p>
-If you needed to wait on multiple different flavors of SRCU
-(but why???), you would need to create a wrapper function resembling
-<tt>call_my_srcu()</tt> for each SRCU flavor.
-
-<table>
-<tr><th> </th></tr>
-<tr><th align="left">Quick Quiz:</th></tr>
-<tr><td>
- But what if I need to wait for multiple RCU flavors, but I also need
- the grace periods to be expedited?
-</td></tr>
-<tr><th align="left">Answer:</th></tr>
-<tr><td bgcolor="#ffffff"><font color="ffffff">
- If you are using expedited grace periods, there should be less penalty
- for waiting on them in succession.
- But if that is nevertheless a problem, you can use workqueues
- or multiple kthreads to wait on the various expedited grace
- periods concurrently.
-</font></td></tr>
-<tr><td> </td></tr>
-</table>
-
-<p>
-Again, it is usually better to adjust the RCU read-side critical sections
-to use a single flavor of RCU, but when this is not feasible, you can use
-<tt>synchronize_rcu_mult()</tt>.
+In <tt>CONFIG_PREEMPT=n</tt> kernels, trampolines cannot be preempted,
+so these APIs map to
+<tt>call_rcu()</tt>,
+<tt>synchronize_rcu()</tt>, and
+<tt>rcu_barrier()</tt>, respectively.
+In <tt>CONFIG_PREEMPT=y</tt> kernels, trampolines can be preempted,
+and these three APIs are therefore implemented by separate functions
+that check for voluntary context switches.
<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
@@ -3248,12 +3180,6 @@ If this becomes a serious problem, it will be necessary to rework the
grace-period state machine so as to avoid the need for the additional
latency.
-<p>
-Expedited grace periods scan the CPUs, so their latency and overhead
-increases with increasing numbers of CPUs.
-If this becomes a serious problem on large systems, it will be necessary
-to do some redesign to avoid this scalability problem.
-
<p>
RCU disables CPU hotplug in a few places, perhaps most notably in the
<tt>rcu_barrier()</tt> operations.
@@ -3298,11 +3224,6 @@ Please note that arrangements that require RCU to remap CPU numbers will
require extremely good demonstration of need and full exploration of
alternatives.
-<p>
-There is an embarrassingly large number of flavors of RCU, and this
-number has been increasing over time.
-Perhaps it will be possible to combine some at some future date.
-
<p>
RCU's various kthreads are reasonably recent additions.
It is quite likely that adjustments will be required to more gracefully
diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index f99cf11b314b..491043fd976f 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -16,12 +16,9 @@ o A CPU looping in an RCU read-side critical section.
o A CPU looping with interrupts disabled.
-o A CPU looping with preemption disabled. This condition can
- result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
- stalls.
+o A CPU looping with preemption disabled.
-o A CPU looping with bottom halves disabled. This condition can
- result in RCU-sched and RCU-bh stalls.
+o A CPU looping with bottom halves disabled.
o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
without invoking schedule(). If the looping in the kernel is
@@ -87,9 +84,9 @@ o A hardware failure. This is quite unlikely, but has occurred
This resulted in a series of RCU CPU stall warnings, eventually
leading the realization that the CPU had failed.
-The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
-warning. Note that SRCU does -not- have CPU stall warnings. Please note
-that RCU only detects CPU stalls when there is a grace period in progress.
+The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning.
+Note that SRCU does -not- have CPU stall warnings. Please note that
+RCU only detects CPU stalls when there is a grace period in progress.
No grace period, no CPU stall warnings.
To diagnose the cause of the stall, inspect the stack traces.
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index c2a7facf7ff9..86d82f7f3500 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -934,7 +934,8 @@ c. Do you need to treat NMI handlers, hardirq handlers,
d. Do you need RCU grace periods to complete even in the face
of softirq monopolization of one or more of the CPUs? For
example, is your code subject to network-based denial-of-service
- attacks? If so, you need RCU-bh.
+ attacks? If so, you should disable softirq across your readers,
+ for example, by using rcu_read_lock_bh().
e. Is your workload too update-intensive for normal use of
RCU, but inappropriate for other synchronization mechanisms?
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9871e649ffef..9e69ec54b7d3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3534,14 +3534,14 @@
In kernels built with CONFIG_RCU_NOCB_CPU=y, set
the specified list of CPUs to be no-callback CPUs.
- Invocation of these CPUs' RCU callbacks will
- be offloaded to "rcuox/N" kthreads created for
- that purpose, where "x" is "b" for RCU-bh, "p"
- for RCU-preempt, and "s" for RCU-sched, and "N"
- is the CPU number. This reduces OS jitter on the
- offloaded CPUs, which can be useful for HPC and
- real-time workloads. It can also improve energy
- efficiency for asymmetric multiprocessors.
+ Invocation of these CPUs' RCU callbacks will be
+ offloaded to "rcuox/N" kthreads created for that
+ purpose, where "x" is "p" for RCU-preempt, and
+ "s" for RCU-sched, and "N" is the CPU number.
+ This reduces OS jitter on the offloaded CPUs,
+ which can be useful for HPC and real-time
+ workloads. It can also improve energy efficiency
+ for asymmetric multiprocessors.
rcu_nocb_poll [KNL]
Rather than requiring that offloaded CPUs
diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index 0f00f9c164ac..23b0c8b20cd1 100644
--- a/Documentation/kernel-per-CPU-kthreads.txt
+++ b/Documentation/kernel-per-CPU-kthreads.txt
@@ -321,7 +321,7 @@ To reduce its OS jitter, do at least one of the following:
to do.
Name:
- rcuob/%d, rcuop/%d, and rcuos/%d
+ rcuop/%d and rcuos/%d
Purpose:
Offload RCU callbacks from the corresponding CPU.