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Subject: Re: sched: Move SCHED_DEBUG sysctl to debugfs
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>
 <YIkp/6/NDL7KsvpY@hirez.programming.kicks-ass.net>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <2326ce94-3707-b099-4fe8-c79547bd8e25@de.ibm.com>
Date:   Wed, 28 Apr 2021 11:31:34 +0200
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On 28.04.21 11:25, Peter Zijlstra wrote:
> On Wed, Apr 28, 2021 at 10:54:37AM +0200, 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.
> 
> The relevant section from origin/master:kernel/sysctl.c:

[...]
> How is migration_cost not under SCHED_DEBUG? The bigger problem is that
> world+dog has SCHED_DEBUG=y in their .config.

Hmm, yes my bad. I disabled it but it was silently reenabled due to a
dependency. So yes you are right, it is under SCHED_DEBUG.