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

From: Joel Fernandes
Date: Mon Oct 24 2022 - 18:09:14 EST

On Mon, Oct 24, 2022 at 1:40 PM Uladzislau Rezki <urezki@xxxxxxxxx> 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@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.
> >
>
> 1. Double-batching?
>
> The kvfree_rcu() interface itself keeps track of when to reclaim:
> a) when a page is full;
> b) when i high storm of freeing over rcu;
> c) when a low memory condition.
>
> such control stays inside the kvfree_rcu(). Converting it to lazy
> variant:
> a) lose the control, what will become as a problem;
> b) nothing is improved.

AFAICS, the only thing being changed is when you are giving memory
back to the system. So you will be holding on to memory a bit longer.
And there's shrinkers that are already flushing those. I don't think
the users of kvfree_rcu() want to free memory instantly. If there is
such usecase, please share it.

> 2. Converting the queue_rcu_work() to lazy variant breaks a humanity
> interpretation when a queued work is supposed to be run. People do not
> expect seconds when they queue the work.

Which people? ;)

> Same as in the kvfree_rcu()
> we do not expect it we even used a high_prio queue in the beginning.
> There are ~10 users who queue the work and they did not expect it to
> be run in 10 seconds when they wrote the code.

That's a bit of a misinterpretation of what I'm saying. A variant
queue_rcu_work_flush() can be added for those users (such as ones that
are not freeing memory).

Thanks.