Date:   Mon, 29 Aug 2022 22:11:39 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Josh Don <joshdon@...gle.com>, Chen Yu <yu.c.chen@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core

Hi Mel, thanks for reviewing!

On 8/29/22 9:08 PM, Mel Gorman Wrote:
> On Tue, Jul 12, 2022 at 04:20:32PM +0800, Abel Wu wrote:
>> When SIS_UTIL is enabled, SIS domain scan will be skipped if
>> the LLC is overloaded. Since the overloaded status is checked
>> in the load balancing at LLC level, the interval is llc_size
>> miliseconds. The duration might be long enough to affect the
>> overall system throughput if idle cores are out of reach in
>> SIS domain scan.
>>
>> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
> 
> Split this patch to move the this_sd lookup into the SIS_PROP section.

OK

> 
> Otherwise, this is the most controversial patch in the series and the
> most likely to cause problems where it wins on some machines and
> workloads and loses on others.
> 
> The corner case to worry about is a workload that is rapidly idling and
> the has_idle_core hint is often wrong. This can happen with hackbench for
> example, or at least this was true when I last looked which is quite some
> time ago. If this hint is often wrong, then there will be full scan cost
> incurred regardless of SIS_UTIL that often fails to find a core.

Yes, I can't agree more. And the situation can become worse when LLC
gets bigger as you mentioned below. I will exclude this part from v2.

> 
> So, first suggestion is to move this patch to the end of the series as
> the other patches are relatively harmless. They could even be merged in
> isolation as a cleanup.
> 
> Second, using the other patches as your baseline, include in the
> changelog what you tested that showed a benefit, what type of machine
> it was and in particular include the number of cores, nodes and the
> span of the LLC.  If you measured any regressions, include that as well
> and make a call on whether you think the patch wins more than it loses.
> The reason to include that information is because the worst corner case
> (all CPUs scanned uselessly) costs more the larger the span of LLC is.
> If all the testing was done on a 2-core SMT-2 machine, the overhead of the
> patch would be negligible but very different if the LLC span is 20 cores.
> While the patch is not obviously wrong, it definitely needs better data,
> Even if you do not have a large test machine available, it's still helpful
> to have it in the changelog because a reviewer like me can decide "this
> needs testing on a larger machine".

Thanks for your detailed suggestions. I will attach benchmark results
along with some analysis next time when posting performance related
patches.

> 
> I did queue this up the entire series for testing and while it sometimes
> benefitted, there were large counter-examples. tbench4 on Zen3 showed some
> large regressions (e.g. 41% loss on 64 clients with a massive increase in
> variability) which has multiple small L3 caches per node. tbench4 (slightly
> modified in mmtests to produce a usable metric) in general showed issues
> across multiple x86-64 machines both AMD and Intel, multiple generations
> with a noticable increase in system CPU usage when the client counts reach
> the stage where the machine is close to saturated. perfpipe for some
> weird reason showed a large regression apparent regresion on Broadwell
> but the variability was also crazy so probably can be ignored. hackbench
> overall looked ok so it's possible I'm wrong about the idle_cores hint
> being often wrong on that workload and I should double check that. It's
> possible the hint is wrong some of the times but right often enough to
> either benefit from using an idle core or by finding an idle sibling which
> may be preferable to stacking tasks on the same CPU.

I attached my test result in one of my replies[1]: netperf showed ~3.5%
regression, hackbench improved a lot, and tbench4 drew. I tested several
times and the results didn't seem vary much.

> 
> The lack of data and the lack of a note on the potential downside is the
> main reason why I'm not acking patch. tbench4 was a particular concern on
> my own tests and it's possible a better patch would be a hybrid approach
> where a limited search of two cores (excluding target) is allowed even if
> SIS_UTIL indicates overload but has_idle_cores is true.
> 

Agreed. And the reason I will exclude this part in v2 is that I plan to
make it part of another feature, SIS filter [2]. The latest version of
SIS filter (not posted yet but soon) will contain all the idle cpus so
we don't need a full scan when has_idle_core. Scanning the filter then
is enough. While it may still cost too much if too many false positive
bits in the filter. Does this direction make sense to you?

[1] 
https://lore.kernel.org/lkml/eaa543fa-421d-2194-be94-6a5e24a33b37@bytedance.com/
[2] 
https://lore.kernel.org/lkml/20220619120451.95251-1-wuyun.abel@bytedance.com/

Thanks & Best Regards,
Abel

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