Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use call_rcu_flush()

From: Paul E. McKenney
Date: Mon Oct 24 2022 - 15:11:05 EST

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@xxxxxxxxx> 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?

Thanx, Paul