Messages in this thread | | | From | Josh Don <> | Date | Tue, 12 Oct 2021 12:45:28 -0700 | Subject | Re: [PATCH] sched/core: forced idle accounting |
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On Tue, Oct 12, 2021 at 5:27 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Oct 11, 2021 at 05:12:43PM -0700, Josh Don wrote: > > > > > + if (WARN_ON_ONCE(!nr_running)) { > > > > + /* can't be forced idle without a running task */ > > > > + } else { > > > > + delta *= nr_forced_idle; > > > > + delta /= nr_running; > > > > + } > > > > > > Now the comment sayeth: > > > > > > > + /* > > > > + * For larger SMT configurations, we need to scale the charged > > > > + * forced idle amount since there can be more than one forced idle > > > > + * sibling and more than one running cookied task. > > > > + */ > > > > > > But why? > > > > 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.
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