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Date:   Fri, 1 Apr 2022 06:01:14 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Eric Dumazet <edumazet@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] rcu-tasks : should take care of sparse cpu masks
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On Thu, Mar 31, 2022 at 09:39:02PM -0700, Eric Dumazet wrote:
> On Thu, Mar 31, 2022 at 5:06 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Thu, Mar 31, 2022 at 04:28:04PM -0700, Eric Dumazet wrote:
> > > On Thu, Mar 31, 2022 at 4:13 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > > >
> > > > The initial setting of ->percpu_enqueue_shift forces all in-range CPU
> > > > IDs to shift down to zero.  The grace-period kthread is allowed to run
> > > > where it likes.  The callback lists are protected by locking, even in
> > > > the case of local access, so this should be safe.
> > > >
> > > > Or am I missing your point?
> > > >
> > >
> > > In fact I have been looking at this code, because we bisected a
> > > regression back to this patch:
> > >
> > > 4fe192dfbe5ba9780df699d411aa4f25ba24cf61 rcu-tasks: Shorten
> > > per-grace-period sleep for RCU Tasks Trace
> > >
> > > It is very possible the regression comes because the RCU task thread
> > > is using more cpu cycles, from 'CPU 0'  where our system daemons are
> > > pinned.
> >
> > Heh!  I did express that concern when creating that patch, but was
> > assured that the latency was much more important.
> >
> > Yes, that patch most definitely increases CPU utilization during RCU Tasks
> > Trace grace periods.  If you can tolerate longer grace-period latencies,
> > it might be worth toning it down a bit.  The ask was for about twice
> > the latency I achieved in my initial attempt, and I made the mistake of
> > forwarding that attempt out for testing.  They liked the shorter latency
> > very much, and objected strenuously to the thought that I might detune
> > it back to the latency that they originally asked for.  ;-)
> >
> > But I can easily provide the means to detune it through use of a kernel
> > boot parameter or some such, if that would help.
> >
> > > But I could not spot where the RCU task kthread is forced to run on CPU 0.
> >
> > I never did intend this kthread be bound anywhere.  RCU's policy is
> > that any binding of its kthreads is the responsibility of the sysadm,
> > be that carbon-based or otherwise.
> >
> > But this kthread is spawned early enough that only CPU 0 is online,
> > so maybe the question is not "what is binding it to CPU 0?" but rather
> > "why isn't something kicking it off of CPU 0?"
> 
> I guess the answer to this question can be found in the following
> piece of code :)
> 
> rcu_read_lock();
> for_each_process_thread(g, t)
>         rtp->pertask_func(t, &holdouts);
> rcu_read_unlock();
> 
> 
> With ~150,000 threads on a 256 cpu host, this holds current cpu for
> very long times:
> 
>  rcu_tasks_trace    11 [017]  5010.544762:
> probe:rcu_tasks_wait_gp: (ffffffff963fb4b0)
>  rcu_tasks_trace    11 [017]  5010.600396:
> probe:rcu_tasks_trace_postscan: (ffffffff963fb7c0)

So about 55 milliseconds for the tasklist scan, correct?  Or am I
losing the plot here?

>  rcu_tasks_trace    11 [022]  5010.618783:
> probe:check_all_holdout_tasks_trace: (ffffffff963fb850)
>  rcu_tasks_trace    11 [022]  5010.618840:
> probe:rcu_tasks_trace_postgp: (ffffffff963fba70)
> 
> In this case, CPU 22 is the victim, not CPU 0 :)

My faith in the scheduler is restored!  ;-)

My position has been that this tasklist scan does not need to be broken
up because it should happen only when a sleepable BPF program is removed,
which is a rare event.

In addition, breaking up this scan is not trivial, because as far as I
know there is no way to force a given task to stay in the list.  I would
have to instead use something like rcu_lock_break(), and restart the
scan if either of the nailed-down pair of tasks was removed from the list.
In a system where tasks were coming and going very frequently, it might
be that such a broken-up scan would never complete.

I can imagine tricks where the nailed-down tasks are kept on a list,
and the nailed-downness is moved to the next task when those tasks
are removed.  I can also imagine a less-than-happy response to such
a proposal.

So I am not currently thinking in terms of breaking up this scan.

Or is there some trick that I am missing?

In the meantime, a simple patch that reduces the frequency of the scan
by a factor of two.  But this would not be the scan of the full tasklist,
but rather the frequency of the calls to check_all_holdout_tasks_trace().
And the total of these looks to be less than 20 milliseconds, if I am
correctly interpreting your trace.  And most of that 20 milliseconds
is sleeping.

Nevertheless, the patch is at the end of this email.

Other than that, I could imagine batching removal of sleepable BPF
programs and using a single grace period for all of their trampolines.
But are there enough sleepable BPF programs ever installed to make this
a useful approach?

Or is the status quo in fact acceptable?  (Hey, I can dream, can't I?)

							Thanx, Paul

> > > I attempted to backport to our kernel all related patches that were
> > > not yet backported,
> > > and we still see a regression in our tests.
> >
> > The per-grace-period CPU consumption of rcu_tasks_trace was intentionally
> > increased by the above commit, and I never have done anything to reduce
> > that CPU consumption.  In part because you are the first to call my
> > attention to it.
> >
> > Oh, and one other issue that I very recently fixed, that has not
> > yet reached mainline, just in case it matters.  If you are building a
> > CONFIG_PREEMPT_NONE=y or CONFIG_PREEMPT_VOLUNTARY=y kernel, but also have
> > CONFIG_RCU_TORTURE_TEST=m (or, for that matter, =y, but please don't in
> > production!), then your kernel will use RCU Tasks instead of vanilla RCU.
> > (Note well, RCU Tasks, not RCU Tasks Trace, the latter being necessaary
> > for sleepable BPF programs regardless of kernel .config).
> >
> > > Please ignore the sha1 in this current patch series, this is only to
> > > show my current attempt to fix the regression in our tree.
> > >
> > > 450b3244f29b rcu-tasks: Don't remove tasks with pending IPIs from holdout list
> > > 5f88f7e9cc36 rcu-tasks: Create per-CPU callback lists
> > > 1a943d0041dc rcu-tasks: Introduce ->percpu_enqueue_shift for dynamic
> > > queue selection
> > > ea5289f12fce rcu-tasks: Convert grace-period counter to grace-period
> > > sequence number
> > > 22efd5093c3b rcu/segcblist: Prevent useless GP start if no CBs to accelerate
> > > 16dee1b3babf rcu: Implement rcu_segcblist_is_offloaded() config dependent
> > > 8cafaadb6144 rcu: Add callbacks-invoked counters
> > > 323234685765 rcu/tree: Make rcu_do_batch count how many callbacks were executed
> > > f48f3386a1cc rcu/segcblist: Add additional comments to explain smp_mb()
> > > 4408105116de rcu/segcblist: Add counters to segcblist datastructure
> > > 4a0b89a918d6 rcu/tree: segcblist: Remove redundant smp_mb()s
> > > 38c0d18e8740 rcu: Add READ_ONCE() to rcu_do_batch() access to rcu_divisor
> > > 0b5d1031b509 rcu/segcblist: Add debug checks for segment lengths
> > > 8a82886fbf02 rcu_tasks: Convert bespoke callback list to rcu_segcblist structure
> > > cbd452a5c01f rcu-tasks: Use spin_lock_rcu_node() and friends
> > > 073222be51f3 rcu-tasks: Add a ->percpu_enqueue_lim to the rcu_tasks structure
> > > 5af10fb0f8fb rcu-tasks: Abstract checking of callback lists
> > > d3e8be598546 rcu-tasks: Abstract invocations of callbacks
> > > 65784460a392 rcu-tasks: Use workqueues for multiple
> > > rcu_tasks_invoke_cbs() invocations
> > > dd6413e355f1 rcu-tasks: Make rcu_barrier_tasks*() handle multiple
> > > callback queues
> > > 2499cb3c438e rcu-tasks: Add rcupdate.rcu_task_enqueue_lim to set
> > > initial queueing
> > > a859f409a503 rcu-tasks: Count trylocks to estimate call_rcu_tasks() contention
> > > 4ab253ca056e rcu-tasks: Avoid raw-spinlocked wakeups from
> > > call_rcu_tasks_generic()
> > > e9a3563fe76e rcu-tasks: Use more callback queues if contention encountered
> > > 4023187fe31d rcu-tasks: Use separate ->percpu_dequeue_lim for callback
> > > dequeueing
> > > 533be3bd47c3 rcu: Provide polling interfaces for Tree RCU grace periods
> > > f7e5a81d7953 rcu-tasks: Use fewer callbacks queues if callback flood ends
> > > bb7ad9078e1b rcu-tasks: Fix computation of CPU-to-list shift counts
> > > d9cebde55539 rcu-tasks: Use order_base_2() instead of ilog2()
> > > 95606f1248f5 rcu-tasks: Set ->percpu_enqueue_shift to zero upon contention


diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 65d6e21a607a..141e2b4c70cc 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1640,10 +1640,10 @@ static int __init rcu_spawn_tasks_trace_kthread(void)
 		rcu_tasks_trace.gp_sleep = HZ / 10;
 		rcu_tasks_trace.init_fract = HZ / 10;
 	} else {
-		rcu_tasks_trace.gp_sleep = HZ / 200;
+		rcu_tasks_trace.gp_sleep = HZ / 100;
 		if (rcu_tasks_trace.gp_sleep <= 0)
 			rcu_tasks_trace.gp_sleep = 1;
-		rcu_tasks_trace.init_fract = HZ / 200;
+		rcu_tasks_trace.init_fract = HZ / 100;
 		if (rcu_tasks_trace.init_fract <= 0)
 			rcu_tasks_trace.init_fract = 1;
 	}