linux-kernel - Re: [PATCH v4 6/7] sched/fair: skip busy cores in SIS search

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Date:   Thu, 30 Jun 2022 18:46:08 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Josh Don <joshdon@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] sched/fair: skip busy cores in SIS search


On 6/30/22 12:16 PM, Chen Yu Wrote:
> On Tue, Jun 28, 2022 at 03:58:55PM +0800, Abel Wu wrote:
>>
>> On 6/27/22 6:13 PM, Abel Wu Wrote:
>> There seems like not much difference except hackbench pipe test at
>> certain groups (30~110).
> OK, smaller LLC domain seems to not have much difference, which might
> suggest that by leveraging load balance code path, the read/write
> to LLC shared mask might not be the bottleneck. I have an vague
> impression that during Aubrey's cpumask searching for idle CPUs
> work[1], there is concern that updating the shared mask in large LLC
> has introduced cache contention and performance degrading. Maybe we
> can find that regressed test case to verify.
> [1] https://lore.kernel.org/all/1615872606-56087-1-git-send-email-aubrey.li@intel.com/

I just went through Aubrey's v1-v11 patches and didn't find any
particular tests other than hackbench/tbench/uperf. Please let
me know if I missed something, thanks!

>> I am intended to provide better scalability
>> by applying the filter which will be enabled when:
>>
>>    - The LLC is large enough that simply traversing becomes
>>      in-sufficient, and/or
>>
>>    - The LLC is loaded that unoccupied cpus are minority.
>>
>> But it would be very nice if a more fine grained pattern works well
>> so we can drop the above constrains.
>>
> We can first try to push a simple version, and later optimize it.
> One concern about v4 is that, we changed the logic in v3, which recorded
> the overloaded CPU, while v4 tracks unoccupied CPUs. An overloaded CPU is
> more "stable" because there are more than 1 running tasks on that runqueue.
> It is more likely to remain "occupied" for a while. That is to say,
> nr_task = 1, 2, 3... will all be regarded as occupied, while only nr_task = 0
> is unoccupied. The former would bring less false negative/positive.

Yes, I like the 'overloaded mask' too, but the downside is extra
cpumask ops needed in the SIS path (the added cpumask_andnot).
Besides, in this patch, the 'overloaded mask' is also unstable due
to the state is maintained at core level rather than per-cpu, some
more thoughts are in cover letter.

> 
> By far I have tested hackbench/schbench/netperf on top of Peter's sched/core branch,
> with SIS_UTIL enabled. Overall it looks good, and netperf has especially
> significant improvement when the load approaches overloaded(which is aligned
> with your comment above). I'll re-run the netperf for several cycles to check the
> standard deviation. And I'm also curious about v3's performance because it
> tracks overloaded CPUs, so I'll also test on v3 with small modifications.

Thanks very much for your reviewing and testing.

Abel