Re: sched: Move SCHED_DEBUG sysctl to debugfs

[Christian Borntraeger]

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.