Re: [PATCH] sched/core: forced idle accounting

[Peter Zijlstra]

On Tue, Oct 12, 2021 at 12:45:28PM -0700, Josh Don wrote:
> On Tue, Oct 12, 2021 at 5:27 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> > > We scale by the number of cpus actually forced idle, since we don't
> > > want to falsely over or under charge forced idle time (defined
> > > strictly as time where we have a runnable task but idle the cpu). The
> > > more important scaling here though is the division over the number of
> > > running entities. This is done so that the aggregate amount of forced
> > > idle over some group of threads makes sense. Ie if we have a cpu with
> > > SMT8, and a group of 7 threads sharing a cookie, we don't want to
> > > accrue 7 units of forced idle time per unit time while the 8th SMT is
> > > forced idle.
> >
> > So why not simply compute the strict per-cpu force-idle time and let
> > userspace sort out the rest?
> 
> Do you mean to compute force idle solely as a per-cpu value? I think
> that would be fine in addition to the per-thread field, but a
> desirable property here is proper attribution to the cause of the
> force idle. That lets system management understand which jobs are the
> most antagonistic from a coresched perspective, and is a signal
> (albeit noisy, due to system state and load balancing decisions) for
> scaling their capacity requirements.

Urgh, reading is hard. I hadn't noticed you did per-task accounting (and
the original changelog doesn't clarify this either).

Also, should all this be undef SCHED_DEBUG ? Or be part of SCHEDSTATS ?