Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity | Date | Wed, 24 Nov 2021 18:04:30 +0000 |
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On 24/11/21 17:58, Vincent Donnefort wrote: > On Wed, Nov 24, 2021 at 05:11:32PM +0000, Valentin Schneider wrote: >> On 24/11/21 14:14, Vincent Donnefort wrote: >> > A shortcut has been introduced in select_idle_sibling() to return prev_cpu >> > if the wakee is woken up by a per-CPU kthread. This is an issue for >> > asymmetric CPU capacity systems where the wakee might not fit prev_cpu >> > anymore. Evaluate asym_fits_capacity() for prev_cpu before using that >> > shortcut. >> > >> > Fixes: 52262ee567ad ("sched/fair: Allow a per-CPU kthread waking a task to stack on the same CPU, to fix XFS performance regression") >> >> Shouldn't that rather be >> >> b4c9c9f15649 ("sched/fair: Prefer prev cpu in asymmetric wakeup path") > > Yes definitely, my bad! > >> >> ? This is an ulterior commit to the one you point to, and before then >> asymmetric CPU systems wouldn't use any of the sis() heuristics. >> >> I reportedly reviewed said commit back then, and don't recall anything >> specific about that conditional... The cover-letter for v2 states: >> >> https://lore.kernel.org/lkml/20201028174412.680-1-vincent.guittot@linaro.org/ >> """ >> don't check capacity for the per-cpu kthread UC because the assumption is >> that the wakee queued work for the per-cpu kthread that is now complete and >> the task was already on this cpu. >> """ >> >> So the assumption here is that current is gonna sleep right after waking up >> p, so current's utilization doesn't matter, and p was already on prev, so >> it should fit there... > > I don't think the assumption that "p was already on prev should fit" is > correct if we take into account uclamp min. That value can change from one > activation to the other and make that task artificially too big for prev_cpu... >
Humph, good point, hadn't thought of that.
>> >> I'm thinking things should actually be OK with your other patch that >> excludes 'current == swapper' from this condition. > > ...But indeed if we add [1] to the equation, this patch here would only > protect against that specific corner case. > > (And probably also against the fact that this same task could have a value > that doesn't fit this CPU anymore but didn't trigger misfit during its previous > activation?)
That would imply crossing the misfit threshold right at the dequeue signal update, but that can happen.
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