From: Seth Forshee <[email protected]>
Currently rcu_barrier() is used to ensure that no readers of the
inactive mini_Qdisc buffer remain before it is reused. This waits for
any pending RCU callbacks to complete, when all that is actually
required is to wait for one RCU grace period to elapse after the buffer
was made inactive. This means that using rcu_barrier() may result in
unnecessary waits.
To improve this, store the current RCU state when a buffer is made
inactive and use poll_state_synchronize_rcu() to check whether a full
grace period has elapsed before reusing it. If a full grace period has
not elapsed, wait for a grace period to elapse, and in the non-RT case
use synchronize_rcu_expedited() to hasten it.
Since this approach eliminates the RCU callback it is no longer
necessary to synchronize_rcu() in the tp_head==NULL case. However, the
RCU state should still be saved for the previously active buffer.
Before this change I would typically see mini_qdisc_pair_swap() take
tens of milliseconds to complete. After this change it typcially
finishes in less than 1 ms, and often it takes just a few microseconds.
Thanks to Paul for walking me through the options for improving this.
Cc: "Paul E. McKenney" <[email protected]>
Signed-off-by: Seth Forshee <[email protected]>
---
v2:
- Rebase to net-next
include/net/sch_generic.h | 2 +-
net/sched/sch_generic.c | 38 +++++++++++++++++++-------------------
2 files changed, 20 insertions(+), 20 deletions(-)
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index ada02c4a4f51..22179b2fda72 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -1302,7 +1302,7 @@ struct mini_Qdisc {
struct tcf_block *block;
struct gnet_stats_basic_sync __percpu *cpu_bstats;
struct gnet_stats_queue __percpu *cpu_qstats;
- struct rcu_head rcu;
+ unsigned long rcu_state;
};
static inline void mini_qdisc_bstats_cpu_update(struct mini_Qdisc *miniq,
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index b0ff0dff2773..24899efc51be 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1487,10 +1487,6 @@ void psched_ppscfg_precompute(struct psched_pktrate *r, u64 pktrate64)
}
EXPORT_SYMBOL(psched_ppscfg_precompute);
-static void mini_qdisc_rcu_func(struct rcu_head *head)
-{
-}
-
void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
struct tcf_proto *tp_head)
{
@@ -1503,28 +1499,30 @@ void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
if (!tp_head) {
RCU_INIT_POINTER(*miniqp->p_miniq, NULL);
- /* Wait for flying RCU callback before it is freed. */
- rcu_barrier();
- return;
- }
+ } else {
+ miniq = !miniq_old || miniq_old == &miniqp->miniq2 ?
+ &miniqp->miniq1 : &miniqp->miniq2;
- miniq = !miniq_old || miniq_old == &miniqp->miniq2 ?
- &miniqp->miniq1 : &miniqp->miniq2;
+ /* We need to make sure that readers won't see the miniq
+ * we are about to modify. So ensure that at least one RCU
+ * grace period has elapsed since the miniq was made
+ * inactive.
+ */
+ if (IS_ENABLED(CONFIG_PREEMPT_RT))
+ cond_synchronize_rcu(miniq->rcu_state);
+ else if (!poll_state_synchronize_rcu(miniq->rcu_state))
+ synchronize_rcu_expedited();
- /* We need to make sure that readers won't see the miniq
- * we are about to modify. So wait until previous call_rcu callback
- * is done.
- */
- rcu_barrier();
- miniq->filter_list = tp_head;
- rcu_assign_pointer(*miniqp->p_miniq, miniq);
+ miniq->filter_list = tp_head;
+ rcu_assign_pointer(*miniqp->p_miniq, miniq);
+ }
if (miniq_old)
- /* This is counterpart of the rcu barriers above. We need to
+ /* This is counterpart of the rcu sync above. We need to
* block potential new user of miniq_old until all readers
* are not seeing it.
*/
- call_rcu(&miniq_old->rcu, mini_qdisc_rcu_func);
+ miniq_old->rcu_state = start_poll_synchronize_rcu();
}
EXPORT_SYMBOL(mini_qdisc_pair_swap);
@@ -1543,6 +1541,8 @@ void mini_qdisc_pair_init(struct mini_Qdisc_pair *miniqp, struct Qdisc *qdisc,
miniqp->miniq1.cpu_qstats = qdisc->cpu_qstats;
miniqp->miniq2.cpu_bstats = qdisc->cpu_bstats;
miniqp->miniq2.cpu_qstats = qdisc->cpu_qstats;
+ miniqp->miniq1.rcu_state = get_state_synchronize_rcu();
+ miniqp->miniq2.rcu_state = miniqp->miniq1.rcu_state;
miniqp->p_miniq = p_miniq;
}
EXPORT_SYMBOL(mini_qdisc_pair_init);
--
2.30.2
From: Seth Forshee <[email protected]>
The only valid values for a miniq pointer are NULL or a pointer to
miniq1 or miniq2, so testing for miniq_old != &miniq1 is functionally
equivalent to testing that it is NULL or equal to &miniq2.
Suggested-by: Jakub Kicinski <[email protected]>
Signed-off-by: Seth Forshee <[email protected]>
---
net/sched/sch_generic.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 24899efc51be..3b0f62095803 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1500,7 +1500,7 @@ void mini_qdisc_pair_swap(struct mini_Qdisc_pair *miniqp,
if (!tp_head) {
RCU_INIT_POINTER(*miniqp->p_miniq, NULL);
} else {
- miniq = !miniq_old || miniq_old == &miniqp->miniq2 ?
+ miniq = miniq_old != &miniqp->miniq1 ?
&miniqp->miniq1 : &miniqp->miniq2;
/* We need to make sure that readers won't see the miniq
--
2.30.2
Hello:
This series was applied to netdev/net-next.git (master)
by Jakub Kicinski <[email protected]>:
On Tue, 26 Oct 2021 08:06:59 -0500 you wrote:
> From: Seth Forshee <[email protected]>
>
> Currently rcu_barrier() is used to ensure that no readers of the
> inactive mini_Qdisc buffer remain before it is reused. This waits for
> any pending RCU callbacks to complete, when all that is actually
> required is to wait for one RCU grace period to elapse after the buffer
> was made inactive. This means that using rcu_barrier() may result in
> unnecessary waits.
>
> [...]
Here is the summary with links:
- [v2] net: sch: eliminate unnecessary RCU waits in mini_qdisc_pair_swap()
https://git.kernel.org/netdev/net-next/c/267463823adb
- [2/1] net: sch: simplify condtion for selecting mini_Qdisc_pair buffer
https://git.kernel.org/netdev/net-next/c/85c0c3eb9a66
You are awesome, thank you!
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