Messages in this thread | | | Date | Tue, 10 Apr 2018 12:53:05 -0700 | From | Tejun Heo <> | Subject | Re: [RFC] mm, slab: reschedule cache_reap() on the same CPU |
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Hello,
On Tue, Apr 10, 2018 at 09:40:19PM +0200, Vlastimil Babka wrote: > On 04/10/2018 04:12 PM, Christopher Lameter wrote: > > On Tue, 10 Apr 2018, Vlastimil Babka wrote: > > > >> cache_reap() is initially scheduled in start_cpu_timer() via > >> schedule_delayed_work_on(). But then the next iterations are scheduled via > >> schedule_delayed_work(), thus using WORK_CPU_UNBOUND. > > > > That is a bug.. cache_reap must run on the same cpu since it deals with > > the per cpu queues of the current cpu. Scheduled_delayed_work() used to > > guarantee running on teh same cpu. > > Did it? When did it stop? (which stable kernels should we backport to?)
It goes back to v4.5 - ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask CPUs") which made WQ_CPU_UNBOUND on percpu workqueues honor wq_unbound_cpusmask so that cpu isolation works better. Unless the force_rr option or unbound_cpumask is set, it still follows local cpu.
> So is my assumption correct that without specifying a CPU, the next work > might be processed on a different cpu than the current one, *and also* > be executed with a kthread/u* that can migrate to another cpu *in the > middle of the work*? Tejun?
For percpu work items, they'll keep executing on the same cpu it started on unless the cpu goes down while executing.
> > schedule_delayed_work_on(smp_processor_id(), work, round_jiffies_relative(REAPTIMEOUT_AC)); > > > > instead all of the other changes? > > If we can rely on that 100%, sure.
Yeah, you can.
Thanks.
-- tejun
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