Re: Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core

[Abel Wu <wuyun.abel@bytedance.com>]

On 8/29/22 10:56 PM, Mel Gorman Wrote:
> On Mon, Aug 29, 2022 at 10:11:39PM +0800, Abel Wu wrote:
>> Hi Mel, thanks for reviewing!
>>
> 
> No problem, sorry it took so long.
> 
>>> 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.
>>
> 
> Thanks, include this in the changelog. While I had different figures for
> hackbench, the figures are still fine. I had similar figures for netperf
> (~3-4% regression on some machines but not universal). The tbench figures
> are interesting because for you, it was mostly neutral but I did test
> it with a CascadeLake machine and had worse results and that machine is
> smaller in terms of core counts than yours.

Interesting, my test machine model is Intel(R) Xeon(R) Platinum 8260 CPU
@ 2.40GHz, which is based on Cascade Lake.  This time I re-tested with
SNC enabled, so each NUMA node now has 12 cores (was 24), but the tbench
result is still neutral..

I just remembered that in the Conclusion part of SIS filter v2 patchset
mentioned a suspicious 50%~90% improvement in netperf&tbench test. Seems
like the result can be misleading under certain conditions.

https://lore.kernel.org/lkml/20220409135104.3733193-1-wuyun.abel@bytedance.com/

> 
>>>
>>> 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/
>>
> 
> The filter idea is tricky, it will always struggle with "accurate
> information" vs "cost of cpumask update" and what you link indicates that
> it's updated on the tick boundary. That will be relatively cheap but could
> mean that searches near the point of saturation or overload will have
> false positives and negatives which you are already aware of given the

I didn't exclude the CPU_NEWLY_IDLE case, yet still the filter worked
the way unexpected as I found recently. The propagation of the idle or
sched-idle cpus can be skipped if rq->avg_idle is small enough (which
is sysctl_sched_migration_cost 500us by default). This can result in
the benchmarks of frequent wakeup type losing some throughput.

So the filter of my latest version will be updated at __update_idle_core
rather than load_balance, and reset when a full domain scan fails. I
will include more details in the log when posting, and I wish you can
give some advice then.

> older series. It does not kill the idea but I would strongly suggest that
> you do the simple thing first. That would yield potentially 3-4 patches
> at the end of the series
> 
> 1. Full scan for cores (this patch effectively)
> 2. Limited scan of 2 cores when SIS_UTIL cuts off but has_idle_cores is true
>     (Compare 1 vs 2 in the changelog, not expected to be a universal win but
>      should have better "worst case" behaviour to be worth merging)
> 3. Filtered scan tracking idle CPUs as a hint while removing the
>     "limited scan"
>     (Compare 2 vs 3)
> 4. The SIS "de-entrophy" patch that tries to cope with false positives
>     and negatives
>     (Compare 3 vs 4)
> 
> That way if the later patches do not work as expected then they can be
> easily dropped without affecting the rest of the series.
> 

Copy. Thanks for your advice!

Best Regards,
Abel