Re: [PATCH] sched/fair: no sync wakeup from interrupt context

Ingo Molnar
2022-08-03T09:18:38Z

* Libo Chen <libo.chen@oracle.com> wrote:

>
>
> On 8/1/22 06:26, Ingo Molnar wrote:
> > * K Prateek Nayak <kprateek.nayak@amd.com> wrote:
> >
> > > Hello Libo and Peter,
> > >
> > > tl;dr
> > >
> > > - We observed a major regression with tbench when testing the latest tip
> > > sched/core at:
> > > commit 14b3f2d9ee8d "sched/fair: Disallow sync wakeup from interrupt context"
> > > Reason for the regression are the fewer affine wakeups that leaves the
> > > client farther away from the data it needs to consume next primed in the
> > > waker's LLC.
> > > Such regressions can be expected from tasks that use sockets to communicate
> > > significant amount of data especially on system with multiple LLCs.
> > >
> > > - Other benchmarks have a comparable behavior to the tip at previous commit
> > > commit : 91caa5ae2424 "sched/core: Fix the bug that task won't enqueue
> > > into core tree when update cookie"
> > >
> > > I'll leave more details below.
> > Mel Gorman also warned about this negative side-effect in:
> >
> > Subject: Re: [PATCH] sched/fair: no sync wakeup from interrupt context
> > Date: Fri, 15 Jul 2022 11:07:38 +0100
> > Message-ID: <20220715100738.GD3493@suse.de>
> >
> > https://urldefense.com/v3/__https://lore.kernel.org/all/20220715100738.GD3493@suse.de/__;!!ACWV5N9M2RV99hQ!PQsIeuK0UwII-A0xS-B3plepNniNeyw14OJowT1cYL-tnuN99MkWfg9C8P60tVFFrnxj0NEanUmEkA$
> ?? Mel was talking about a completely different thing, I brought up a
> different patch that I wanted to revert and Mel thought it would hurt other
> workloads which don't benefit from pulling but
> as you can see, tbench somehow benefits from it, at least according to one
> metric from one workload.

Yeah - but nevertheless the discussion with Mel was open-ended AFAICS, and
the 'major tbench regression' report by K Prateek Nayak above still stands
and needs to be investigated/understood, right?

Thanks,

Ingo