Re: sched: Move SCHED_DEBUG sysctl to debugfs

Christian Borntraeger
2021-04-28T09:32:22Z

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.