Messages in this thread | | | Date | Tue, 12 Oct 2021 14:27:08 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/core: forced idle accounting |
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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?
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