Messages in this thread | | | From | Xuewen Yan <> | Date | Thu, 9 Nov 2023 14:38:19 +0800 | Subject | Re: [PATCH v6 18/20] sched: Handle blocked-waiter migration (and return migration) |
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On Thu, Nov 9, 2023 at 2:08 PM John Stultz <jstultz@google.com> wrote: > > On Wed, Nov 8, 2023 at 9:32 PM Xuewen Yan <xuewen.yan94@gmail.com> wrote: > > I understand what you mean here. But I have some other worries: > > considering the scenario of Big-Little cpu topology, when EAS is in > > effect. > > If the owner is a "small task", and the owner is on a small core, and > > the blocked task is a "big task", the block task will be directly > > migrated to the small core, > > and because the task is on rq, when wake up, it would skip the > > “select_task_rq” and put it directly on the small core. > > As a result, the big task's performance may decrease. > > The same reason, a small task may be placed on the big core, and there > > may be a regression in power consumption. > > > ... > > > +static inline bool proxy_return_migration(struct rq *rq, struct rq_flags *rf, > > > + struct task_struct *next) > > > +{ > > > + if (!sched_proxy_exec()) > > > + return false; > > > + > > > + if (next->blocked_on && next->blocked_on_waking) { > > > + if (!is_cpu_allowed(next, cpu_of(rq))) { > > > > > > Based on the above reasons, could this be changed to the following? > > /* When EAS enabled, we hope the task selects the cpu again */ > > if (sched_energy_enabled() || !is_cpu_allowed(next, > > cpu_of(rq)) ) > > Hey! Thanks so much for the feedback and review! > > That is a good point, this would cause a misplacement on the lock > handoff. Though I fret having to run through the return migration > lock juggling here for every blocked_on wakeup would further hurt > performance as well. > > I'm currently trying to see if I can extend the blocked_on_waking flag > to keep more state (BLOCKED, WAKING, RUNNABLE) so that we can move the > return migration back to the the try_to_wake_up() call path, while > avoiding the task from becoming suddenly runnable on wakeup while on > the wrong runqueue. This would avoid the lock juggling as we'd > already have the pi_lock. Though I'm a little hesitant as doing the > deactivate()/select_task_rq()/activate() steps from ttwu might muddle > up the careful logic around the on_rq/ttwu_runnable checks (definitely > had issues in that area with earlier versions of the patch).
I also think it is better to put the return migration back to the try_to_wake_up() call path. When mutex_unlock, could we deactivate the block task before adding it to wake_q? In this case, it can follow the try_to_wake_up patch. But at this time, the trace_sched_blocked_reason may be no need?
> > > In addition, I also thought that since the block task is no longer > > dequeued, this will definitely cause the load on the CPU to increase. > > Perhaps we need to evaluate the impact of this on power consumption. > > > > Yeah. I've got that still as a todo in the cover letter: > * CFS load balancing. Blocked tasks may carry forward load (PELT) > to the lock owner's CPU, so CPU may look like it is overloaded. > > If you have any thoughts there for a preferred approach, I'd be happy to hear.
Okay, I'm still studying these patches carefully, and I will to test these patches later. When I find other problems, I will be happy to share.
Thanks!
> > thanks > -john
BR --- xuewen
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