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Subject: Re: sched: Move SCHED_DEBUG sysctl to debugfs
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
References: <20210412102001.287610138@infradead.org>
 <20210427145925.5246-1-borntraeger@de.ibm.com>
 <YIkgzUWEPaXQTCOv@hirez.programming.kicks-ass.net>
 <cf2a6c6c-21ea-df7b-94d1-940a344b8d26@de.ibm.com>
Message-ID: <49af1ed4-26b1-071e-365e-f701edb0eaa7@de.ibm.com>
Date:   Wed, 28 Apr 2021 10:58:24 +0200
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On 28.04.21 10:54, Christian Borntraeger wrote:
> 
> 
> On 28.04.21 10:46, Peter Zijlstra wrote:
>> 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.
> 
> No its not. sched_migration_cost_ns was NEVER protected by CONFIG_SCHED_DEBUG.
> It was available on all kernels with CONFIG_SMP.

Have to correct myself, it was SCHED_DEBUG in 3.0.
> 
>>
>> 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.
> 
> Yes, I agree. We have seen the effect of this value recently and we want
> look into that. Still that does not change the fact that you are removing
> an interface that was there for ages.