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From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: bristot@redhat.com, bsegall@google.com, dietmar.eggemann@arm.com,
	greg@kroah.com, gregkh@linuxfoundation.org, joshdon@google.com,
	juri.lelli@redhat.com, linux-kernel@vger.kernel.org,
	linux@rasmusvillemoes.dk, mgorman@suse.de, mingo@kernel.org,
	rostedt@goodmis.org, valentin.schneider@arm.com,
	vincent.guittot@linaro.org, linux-s390@vger.kernel.org,
	kvm@vger.kernel.org
Subject: Re: sched: Move SCHED_DEBUG sysctl to debugfs
Date: Wed, 28 Apr 2021 16:49:44 +0200	[thread overview]
Message-ID: <89ccb344-7a03-65e6-826d-4807e1ab2815@de.ibm.com> (raw)
In-Reply-To: <YIlXQ43b6+7sUl+f@hirez.programming.kicks-ass.net>

On 28.04.21 14:38, Peter Zijlstra wrote:
> On Wed, Apr 28, 2021 at 11:42:57AM +0200, Christian Borntraeger wrote:
>> On 28.04.21 10:46, Peter Zijlstra wrote:
>> [..]
>>> The right thing to do here is to analyze the situation and determine why
>>> migration_cost needs changing; is that an architectural thing, does s390
>>> benefit from less sticky tasks due to its cache setup (the book caches
>>> could be absorbing some of the penalties here for example). Or is it
>>> something that's workload related, does KVM intrinsically not care about
>>> migrating so much, or is it something else.
>>
>> So lets focus on the performance issue.
>>
>> One workload where we have seen this is transactional workload that is
>> triggered by external network requests. So every external request
>> triggered a wakup of a guest and a wakeup of a process in the guest.
>> The end result was that KVM was 40% slower than z/VM (in terms of
>> transactions per second) while we had more idle time.
>> With smaller sched_migration_cost_ns (e.g. 100000) KVM was as fast
>> as z/VM.
>>
>> So to me it looks like that the wakeup and reschedule to a free CPU
>> was just not fast enough. It might also depend where I/O interrupts
>> land. Not sure yet.
> 
> So there's unfortunately three places where migration_cost is used; one
> is in {nohz_,}newidle_balance(), see below. Someone tried removing it
> before and that ran into so weird regressions somewhere. But it is worth
> checking if this is the thing that matters for your workload.
> 
> The other (main) use is in task_hot(), where we try and prevent
> migrating tasks that have recently run on a CPU. We already have an
> exception for SMT there, because SMT siblings share all cache levels per
> defintion, so moving it to the sibling should have no ill effect.
> 
> It could be that the current measure is fundamentally too high for your
> machine -- it is basically a random number that was determined many
> years ago on some random x86 machine, so it not reflecting reality today
> on an entirely different platform is no surprise.
> 
> Back in the day, we had some magic code that measured cache latency per
> sched_domain and we used that, but that suffered from boot-to-boot
> variance and made things rather non-deterministic, but the idea of
> having per-domain cost certainly makes sense.
> 
> Over the years people have tried bringing parts of that back, but it
> never really had convincing numbers justifying the complexity. So that's
> another thing you could be looking at I suppose.
> 
> And then finally we have an almost random use in rebalance_domains(),
> and I can't remember the story behind that one :/
> 
> 
> Anyway, TL;DR, try and figure out which of these three is responsible
> for your performance woes. If it's the first, the below patch might be a
> good candidate. If it's task_hot(), we might need to re-eval per domain
> costs. If its that other thing, I'll have to dig to figure out wth that
> was supposed to accomplish ;-)

Thanks for the insight. I will try to find out which of these areas make
a difference here.
[..]

  reply	other threads:[~2021-04-28 14:50 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-12 10:14 [PATCH v2 0/9] sched: Clean up SCHED_DEBUG Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 1/9] sched/numa: Allow runtime enabling/disabling of NUMA balance without SCHED_DEBUG Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 2/9] sched: Remove sched_schedstats sysctl out from under SCHED_DEBUG Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 3/9] sched: Dont make LATENCYTOP select SCHED_DEBUG Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] sched: Don't " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 4/9] sched: Move SCHED_DEBUG sysctl to debugfs Peter Zijlstra
2021-04-15 16:29   ` [PATCH] sched/debug: Rename the sched_debug parameter to sched_debug_verbose Peter Zijlstra
2021-04-19 19:26     ` Josh Don
2021-04-16 15:53   ` [tip: sched/core] sched: Move SCHED_DEBUG sysctl to debugfs tip-bot2 for Peter Zijlstra
2021-04-27 14:59   ` Christian Borntraeger
2021-04-27 15:09     ` Steven Rostedt
2021-04-27 15:17       ` Christian Borntraeger
2021-04-28  8:47       ` Peter Zijlstra
2021-04-28  8:46     ` Peter Zijlstra
2021-04-28  8:54       ` Christian Borntraeger
2021-04-28  8:58         ` Christian Borntraeger
2021-04-28  9:25         ` Peter Zijlstra
2021-04-28  9:31           ` Christian Borntraeger
2021-04-28  9:42       ` Christian Borntraeger
2021-04-28 12:38         ` Peter Zijlstra
2021-04-28 14:49           ` Christian Borntraeger [this message]
2021-07-07 12:34           ` [PATCH 0/1] Improve yield (was: sched: Move SCHED_DEBUG sysctl to debugfs) Christian Borntraeger
2021-07-07 12:34             ` [PATCH 1/1] sched/fair: improve yield_to vs fairness Christian Borntraeger
2021-07-07 18:07               ` kernel test robot
2021-07-23  9:35               ` Mel Gorman
2021-07-23 12:36                 ` Christian Borntraeger
2021-07-23 16:21                   ` Mel Gorman
2021-07-26 18:41                     ` Christian Borntraeger
2021-07-26 19:32                       ` Mel Gorman
2021-07-27  6:59                         ` Christian Borntraeger
2021-07-27 18:57                       ` Benjamin Segall
2021-07-28 16:23                         ` Christian Borntraeger
2021-08-10  8:49                           ` Vincent Guittot
2021-07-27 13:29                     ` Peter Zijlstra
2021-07-27 13:33                 ` Peter Zijlstra
2021-07-27 14:31                   ` Mel Gorman
2021-04-12 10:14 ` [PATCH v2 5/9] sched,preempt: Move preempt_dynamic to debug.c Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 6/9] debugfs: Implement debugfs_create_str() Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 7/9] sched,debug: Convert sysctl sched_domains to debugfs Peter Zijlstra
2021-04-13 14:55   ` Valentin Schneider
2021-04-15  9:06     ` Peter Zijlstra
2021-04-15 12:16       ` Dietmar Eggemann
2021-04-15 12:34       ` Valentin Schneider
2021-04-15 13:02         ` Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 8/9] sched: Move /proc/sched_debug " Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 9/9] sched,fair: Alternative sched_slice() Peter Zijlstra
2021-04-12 10:26   ` Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra

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