Messages in this thread | | | Date | Tue, 21 Jan 2014 20:37:59 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 8/9] sched/fair: Optimize cgroup pick_next_task_fair |
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On Tue, Jan 21, 2014 at 11:24:39AM -0800, bsegall@google.com wrote: > Peter Zijlstra <peterz@infradead.org> writes:
> > +#ifdef CONFIG_FAIR_GROUP_SCHED > > + /* > > + * If we haven't yet done put_prev_entity and the selected task is > > + * a different task than we started out with, try and touch the least > > + * amount of cfs_rq trees. > > + */ > > + if (prev) { > > + if (prev != p) { > > + pse = &prev->se; > > + > > + while (!(cfs_rq = is_same_group(se, pse))) { > > + int se_depth = se->depth; > > + int pse_depth = pse->depth; > > + > > + if (se_depth <= pse_depth) { > > + put_prev_entity(cfs_rq_of(pse), pse); > > + pse = parent_entity(pse); > > + } > > + if (se_depth >= pse_depth) { > > + set_next_entity(cfs_rq_of(se), se); > > + se = parent_entity(se); > > + } > > + } > > > > + put_prev_entity(cfs_rq, pse); > > + set_next_entity(cfs_rq, se); > > + }
(A)
> > + /* > > + * In case the common cfs_rq got throttled, just give up and > > + * put the stack and retry. > > + */ > > + if (unlikely(check_cfs_rq_runtime(cfs_rq))) { > > + put_prev_task_fair(rq, p); > > + prev = NULL; > > + goto again; > > + } > > This double-calls put_prev_entity on any non-common cfs_rqs and ses, > which means double __enqueue_entity, among other things. Just doing the > put_prev loop from se->parent should fix that.
I'm not seeing that, so at point (A) we've completely switched over from @prev to @p, we've put all pse until the common parent and set all se back to @p.
So if we then do: put_prev_task_fair(rq, p), we simply undo all the set_next_entity(se) we just did, and continue from the common parent upwards.
> However, any sort of abort means that we may have already done > set_next_entity on some children, which even with the changes to > pick_next_entity will cause problems, up to and including double > __dequeue_entity I think.
But the abort is only done after we've completely set up @p as the current task.
Yes, completely tearing it down again is probably a waste, but given that bandwidth enforcement should be rare and I didn't want to complicate things even further for rare cases.
> Also, this way we never do check_cfs_rq_runtime on any parents of the > common cfs_rq, which could even have been the reason for the resched to > begin with. I'm not sure if there would be any problem doing it on the > way down or not, I don't see any problems at a glance.
Oh, so we allow a parent to have less runtime than the sum of all its children?
Indeed, in that case we can miss something... we could try to call check_cfs_rq_runtime() from the initial top-down selection loop? When true, just put the entire stack and don't pretend to be smart?
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