Messages in this thread | | | Date | Fri, 2 Nov 2018 09:53:14 +0000 | From | Patrick Bellasi <> | Subject | Re: [PATCH] sched/fair: util_est: fix cpu_util_wake for execl |
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On 31-Oct 19:45, Peter Zijlstra wrote: > On Tue, Oct 30, 2018 at 04:09:47PM +0000, Patrick Bellasi wrote: > > > Let's fix this by ensuring to always discount the task estimated > > utilization from the CPU's estimated utilization when the task is also > > the current one. The same benchmark of the bug report, executed on a > > dual socket 40 CPUs Intel(R) Xeon(R) CPU E5-2690 v2 @ 3.00GHz machine, > > reports these "Execl Throughput" figures (higher the better): > > Before this we have: > > /* Discount task's blocked util from CPU's util */ > util -= min_t(unsigned int, util, task_util(p)); > > at the very least that comment is now inaccurate, since @p might not be > blocked.
Right... will fix this too.
> > @@ -6258,8 +6267,17 @@ static unsigned long cpu_util_wake(int cpu, struct task_struct *p) > > * covered by the following code when estimated utilization is > > * enabled. > > */ > > - if (sched_feat(UTIL_EST)) > > - util = max(util, READ_ONCE(cfs_rq->avg.util_est.enqueued)); > > + if (sched_feat(UTIL_EST)) { > > + unsigned int estimated = > > + READ_ONCE(cfs_rq->avg.util_est.enqueued); > > + > > + if (unlikely(current == p || task_on_rq_queued(p))) { > > I'm confused by the need for 'current == p', afaict task_on_rq_queued(p) > is sufficient -- we've already established task_cpu(p) == cpu earlier.
Mmm... you right, I've got confused by the fact that current is removed from the RBTree, but we keep tracking it as:
on_rq = TASK_ON_RQ_QUEUED
... unless, select_task_rq_fair() races with LB's:
detach_task() p->on_rq = TASK_ON_RQ_MIGRATING; -----------------------------------A deactivate_task() \ dequeue_task() +- RaceTime util_est_dequeue() / -----------------------------------B set_task_cpu() migrate_task_rq{_fair}() detach_entity_cfs_rq()
where, in [A..B] we will still avoid to discount *p's estimated utilization. :/
Do you think we can live with that for the time being, maybe by just adding a comment, or should we try to close that too ?
Eventually, the (current == p) check, maybe moved to the right of the OR condition above, should certainly close the race window for the specific UnixBench's execl case. Assuming for example the execl is executed by a misfit task which is target of an active load balance...
> > + estimated -= min_t(unsigned int, estimated, > > + (_task_util_est(p) | UTIL_AVG_UNCHANGED)); > > + } > > + > > + util = max(util, estimated); > > + } > > Also, I think it is about time we find a suitable name for: > > #define xxx(_var, _val) do { \
remove_contrib(_var, _val) ?
> typeof(_var) var = (_var); \ > typeof(_var) val = (_val); \ > typeof(_var) res = var - val; \ > if (res > var) \ > res = 0; \ > (_var) = res; \ > } while (0) > > Which is basically sub_positive() but without the READ_ONCE/WRITE_ONCE > stuff.
Perhaps there are still some paths in where sub_positive() can be recycled... will look better into that and see what we can do on that polishing side. However, I'll keep all that in a different patch.
> We do that: > > var -= min_t(typeof(var), var, val); > > pattern _all_ over.
Cheers Patrick
-- #include <best/regards.h>
Patrick Bellasi
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