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From: Peter Zijlstra <peterz@infradead.org>
To: Christian Borntraeger <borntraeger@de.ibm.com>
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 10:46:05 +0200	[thread overview]
Message-ID: <YIkgzUWEPaXQTCOv@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210427145925.5246-1-borntraeger@de.ibm.com>

On Tue, Apr 27, 2021 at 04:59:25PM +0200, Christian Borntraeger wrote:
> Peter,
> 
> I just realized that we moved away sysctl tunabled to debugfs in next.
> We have seen several cases where it was benefitial to set
> sched_migration_cost_ns to a lower value. For example with KVM I can
> easily get 50% more transactions with 50000 instead of 500000. 
> Until now it was possible to use tuned or /etc/sysctl.conf to set
> these things permanently. 
> 
> Given that some people do not want to have debugfs mounted all the time
> I would consider this a regression. The sysctl tunable was always 
> available.
> 
> I am ok with the "informational" things being in debugfs, but not
> the tunables. So how do we proceed here?

It's all SCHED_DEBUG; IOW you're relying on DEBUG infrastructure for
production performance, and that's your fail.

I very explicitly do not care to support people that poke random values
into those 'tunables'. If people wants to do that, they get to keep any
and all pieces.

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.

Basically, you get to figure out what the actual performance issue is,
and then we can look at what to do about it so that everyone benefits,
and not grow some random tweaks on the interweb that might or might not
actually work for someone else.

  parent reply	other threads:[~2021-04-28  8:48 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 [this message]
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
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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