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Date:   Mon, 24 Oct 2022 13:19:53 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org
Subject: Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use
 call_rcu_flush()
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On Mon, Oct 24, 2022 at 04:12:59PM -0400, Joel Fernandes wrote:
> On Mon, Oct 24, 2022 at 1:36 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Mon, Oct 24, 2022 at 01:20:26PM -0400, Joel Fernandes wrote:
> > > On Mon, Oct 24, 2022 at 1:08 PM Uladzislau Rezki <urezki@gmail.com> wrote:
> > > >
> > > > On Mon, Oct 24, 2022 at 06:55:16PM +0200, Uladzislau Rezki wrote:
> > > > > On Mon, Oct 24, 2022 at 09:48:19AM -0700, Paul E. McKenney wrote:
> > > > > > On Mon, Oct 24, 2022 at 06:25:30PM +0200, Uladzislau Rezki wrote:
> > > > > > > >
> > > > > > > > You guys might need to agree on the definition of "good" here.  Or maybe
> > > > > > > > understand the differences in your respective platforms' definitions of
> > > > > > > > "good".  ;-)
> > > > > > > >
> > > > > > > Indeed. Bad is when once per-millisecond infinitely :) At least in such use
> > > > > > > workload a can detect a power delta and power gain. Anyway, below is a new
> > > > > > > trace where i do not use "flush" variant for the kvfree_rcu():
> > > > > > >
> > > > > > > <snip>
> > > > > > > 1. Home screen swipe:
> > > > > > >          rcuop/0-15      [003] d..1  1792.767750: rcu_batch_start: rcu_preempt CBs=1003 bl=10
> > > > > > >          rcuop/2-33      [002] d..1  1792.771717: rcu_batch_start: rcu_preempt CBs=934 bl=10
> > > > > > >          rcuop/3-40      [001] d..1  1794.811816: rcu_batch_start: rcu_preempt CBs=1508 bl=11
> > > > > > >          rcuop/1-26      [003] d..1  1797.116382: rcu_batch_start: rcu_preempt CBs=2127 bl=16
> > > > > > >          rcuop/4-48      [001] d..1  1797.124422: rcu_batch_start: rcu_preempt CBs=95 bl=10
> > > > > > >          rcuop/5-55      [002] d..1  1797.124731: rcu_batch_start: rcu_preempt CBs=143 bl=10
> > > > > > >          rcuop/6-62      [005] d..1  1798.911719: rcu_batch_start: rcu_preempt CBs=132 bl=10
> > > > > > >          rcuop/2-33      [002] d..1  1803.003966: rcu_batch_start: rcu_preempt CBs=3797 bl=29
> > > > > > >          rcuop/0-15      [003] d..1  1803.004707: rcu_batch_start: rcu_preempt CBs=2969 bl=23
> > >
> > > > > > > 2. App launches:
> > > > > > >          rcuop/4-48      [005] d..1  1831.087612: rcu_batch_start: rcu_preempt CBs=6141 bl=47
> > > > > > >          rcuop/7-69      [007] d..1  1831.095578: rcu_batch_start: rcu_preempt CBs=5464 bl=42
> > > > > > >          rcuop/5-55      [004] d..1  1832.703571: rcu_batch_start: rcu_preempt CBs=8461 bl=66
> > > > > > >          rcuop/0-15      [004] d..1  1833.731603: rcu_batch_start: rcu_preempt CBs=2548 bl=19
> > > > > > >          rcuop/1-26      [006] d..1  1833.743691: rcu_batch_start: rcu_preempt CBs=2567 bl=20
> > > > > > >          rcuop/2-33      [006] d..1  1833.744005: rcu_batch_start: rcu_preempt CBs=2359 bl=18
> > > > > > >          rcuop/3-40      [006] d..1  1833.744286: rcu_batch_start: rcu_preempt CBs=3681 bl=28
> > > > > > >          rcuop/4-48      [002] d..1  1838.079777: rcu_batch_start: rcu_preempt CBs=10444 bl=81
> > > > > > >          rcuop/7-69      [001] d..1  1838.080375: rcu_batch_start: rcu_preempt CBs=12572 bl=98
> > > > > > >            <...>-62      [002] d..1  1838.080646: rcu_batch_start: rcu_preempt CBs=14135 bl=110
> > > > > > >          rcuop/6-62      [000] d..1  1838.087722: rcu_batch_start: rcu_preempt CBs=10839 bl=84
> > > > > > >            <...>-62      [003] d..1  1839.227022: rcu_batch_start: rcu_preempt CBs=1834 bl=14
> > > > > > >            <...>-26      [001] d..1  1839.963315: rcu_batch_start: rcu_preempt CBs=5769 bl=45
> > > > > > >          rcuop/2-33      [001] d..1  1839.966485: rcu_batch_start: rcu_preempt CBs=3789 bl=29
> > > > > > >            <...>-40      [001] d..1  1839.966596: rcu_batch_start: rcu_preempt CBs=6425 bl=50
> > > > > > >          rcuop/2-33      [005] d..1  1840.541272: rcu_batch_start: rcu_preempt CBs=825 bl=10
> > > > > > >          rcuop/2-33      [005] d..1  1840.547724: rcu_batch_start: rcu_preempt CBs=44 bl=10
> > > > > > >          rcuop/2-33      [005] d..1  1841.075759: rcu_batch_start: rcu_preempt CBs=516 bl=10
> > > > > > >          rcuop/0-15      [002] d..1  1841.695716: rcu_batch_start: rcu_preempt CBs=6312 bl=49
> > > > > > >          rcuop/0-15      [003] d..1  1841.709714: rcu_batch_start: rcu_preempt CBs=39 bl=10
> > > > > > >          rcuop/5-55      [004] d..1  1843.112442: rcu_batch_start: rcu_preempt CBs=16007 bl=125
> > > > > > >          rcuop/5-55      [004] d..1  1843.115444: rcu_batch_start: rcu_preempt CBs=7901 bl=61
> > > > > > >          rcuop/6-62      [001] dn.1  1843.123983: rcu_batch_start: rcu_preempt CBs=8427 bl=65
> > > > > > >          rcuop/6-62      [006] d..1  1843.412383: rcu_batch_start: rcu_preempt CBs=981 bl=10
> > > > > > >          rcuop/0-15      [003] d..1  1844.659812: rcu_batch_start: rcu_preempt CBs=1851 bl=14
> > > > > > >          rcuop/0-15      [003] d..1  1844.667790: rcu_batch_start: rcu_preempt CBs=135 bl=10
> > >
> > > Definitely better, but I'd still ask why not just rely on the lazy
> > > batching that we now have, since it is a memory pressure related
> > > usecase. Or another approach could be, for CONFIG_RCU_LAZY, don't
> > > disturb the lazy-RCU batching by queuing these "free memory" CBs; and
> > > instead keep your improved kvfree_rcu() batching only for
> > > !CONFIG_RCU_LAZY.
> >
> > Given that making the kvfree_rcu()-level batching conditional on
> > CONFIG_RCU_LAZY would complicate the code, what bad thing happens when
> > keeping the kvfree_rcu-level batching unconditionally?
> 
> The bad thing happening is power impact. There is a noticeable impact
> in our testing, and when we dropped this particular patch, it got much
> better results.
> 
> I also run rcutop and I see without the patch that I have several
> seconds of laziness at a time, unlike with the patch.

Fair point, but is this visible at the power meter?

							Thanx, Paul

> Even in the beginning when I came up with an implementation for
> call_rcu_lazy(), I had to mark queue_rcu_work() as lazy as well since
> it was quite frequent (on ChromeOS). But when we introduced the
> flush() API, I forgot to not use flush() on it.  But unfortunately
> this patch slipped into my last series when Vlad and I were debugging
> the SCSI issue, and did not really help for the SCSI issue itself.
> 
> Thanks,
> 
>  - Joel