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SubjectRe: [PATCH v2 1/1] mm/vmalloc: Move draining areas out of caller context
On Tue, Jan 25, 2022 at 06:46:35PM +0000, Matthew Wilcox wrote:
> On Tue, Jan 25, 2022 at 06:12:48PM +0100, Uladzislau Rezki wrote:
> > On Tue, Jan 25, 2022 at 04:50:14PM +0000, Matthew Wilcox wrote:
> > > On Tue, Jan 25, 2022 at 05:39:12PM +0100, Uladzislau Rezki (Sony) wrote:
> > > > @@ -1768,7 +1776,8 @@ static void free_vmap_area_noflush(struct vmap_area *va)
> > > >
> > > > /* After this point, we may free va at any time */
> > > > if (unlikely(nr_lazy > lazy_max_pages()))
> > > > - try_purge_vmap_area_lazy();
> > > > + if (!atomic_xchg(&drain_vmap_work_in_progress, 1))
> > > > + schedule_work(&drain_vmap_work);
> > > > }
> > >
> > > Is it necessary to have drain_vmap_work_in_progress? The documentation
> > > says:
> > >
> > > * This puts a job in the kernel-global workqueue if it was not already
> > > * queued and leaves it in the same position on the kernel-global
> > > * workqueue otherwise.
> > >
> > > and the implementation seems to use test_and_set_bit() to ensure this
> > > is true.
> > >
> > It checks pending state, if the work is in run-queue you can place it
> > one more time. The motivation of having it is to prevent the drain work
> > of being placed several times at once what i see on my stress testing.
> >
> > CPU_1: invokes vfree() -> queues the drain work -> TASK_RUNNING
> > CPU_2: invokes vfree() -> queues the drain work one more time since it was not pending
>
> But why not unconditionally call schedule_work() here?
>
We can :) The question is do we agree that extra queuing will be kind of
spurious? Because the CPU_1 will complete all cleanups once it is physically
on CPU and others workers just bail out.

We can disregard those spurious wake-ups for sure. If someone complains about
it in the future we can think later then.

Re-spin and do it unconditionally? I do not have a strong opinion about it.

--
Vlad Rezki

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