linux-kernel - Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use call_rcu

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Message-ID: <Y1a8ei1h7SzyYZx9@pc636>
Date:   Mon, 24 Oct 2022 18:25:30 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org
Subject: Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use
 call_rcu_flush()

>
> 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
<snip>

it is much more better. But. As i wrote earlier there is a patch that i have submitted
some time ago improving kvfree_rcu() batching:

<snip>
commit 51824b780b719c53113dc39e027fbf670dc66028
Author: Uladzislau Rezki (Sony) <urezki@...il.com>
Date:   Thu Jun 30 18:33:35 2022 +0200

    rcu/kvfree: Update KFREE_DRAIN_JIFFIES interval

    Currently the monitor work is scheduled with a fixed interval of HZ/20,
    which is roughly 50 milliseconds. The drawback of this approach is
    low utilization of the 512 page slots in scenarios with infrequence
    kvfree_rcu() calls.  For example on an Android system:
<snip>

The trace that i posted was taken without it.

--
Uladzislau Rezki