linux-kernel - Re: [RFC PATCH] sched/fair: Skip idle CPU search on busy system

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Message-ID: <5f0824ea-d7b6-61f5-db89-1727bae83595@linux.vnet.ibm.com>
Date:   Thu, 10 Aug 2023 21:14:06 +0530
From:   Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To:     Vishal Chourasia <vishalc@...ux.ibm.com>
Cc:     peterz@...radead.org, vincent.guittot@...aro.org,
        srikar@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        mingo@...hat.com, dietmar.eggemann@....com, mgorman@...e.de
Subject: Re: [RFC PATCH] sched/fair: Skip idle CPU search on busy system



On 8/10/23 12:14 AM, Vishal Chourasia wrote:
> On Wed, Jul 26, 2023 at 03:06:12PM +0530, Shrikanth Hegde wrote:
>> When the system is fully busy, there will not be any idle CPU's.
>>
> Tested this patchset on top of v6.4
[...]
> 5 Runs of stress-ng (100% load) on a system with 16CPUs spawning 23 threads for
> 60 minutes.
> 
> stress-ng: 16CPUs, 23threads, 60mins
> 
> - 6.4.0
> 
> | completion time(sec) |      user |        sys |
> |----------------------+-----------+------------|
> |              3600.05 |  57582.44 |       0.70 |
> |              3600.10 |  57597.07 |       0.68 |
> |              3600.05 |  57596.65 |       0.47 |
> |              3600.04 |  57596.36 |       0.71 |
> |              3600.06 |  57595.32 |       0.42 |
> |              3600.06 | 57593.568 |      0.596 | average
> |          0.046904158 | 12.508392 | 0.27878307 | stddev
> 
> - 6.4.0+ (with patch)
> 
> | completion time(sec) |      user |         sys |
> |----------------------+-----------+-------------|
> |              3600.04 |  57596.58 |        0.50 |
> |              3600.04 |  57595.19 |        0.48 |
> |              3600.05 |  57597.39 |        0.49 |
> |              3600.04 |  57596.64 |        0.53 |
> |              3600.04 |  57595.94 |        0.43 |
> |             3600.042 | 57596.348 |       0.486 | average
> |         0.0089442719 | 1.6529610 | 0.072938330 | stddev
> 
> The average system time is slightly lower in the patched version (0.486 seconds)
> compared to the 6.4.0 version (0.596 seconds). 
> The standard deviation for system time is also lower in the patched version
> (0.0729 seconds) than in the 6.4.0 version (0.2788 seconds), suggesting more
> consistent system time results with the patch.
> 
> vishal.c

Thank you very much Vishal for trying this out. 

Meanwhile, I am yet to try the suggestion given by chen. Let me see if that works okay.