Re: [PATCH 02/10] workqueue: use cpu_possible_mask instead of cpu_active_mask to break affinity

[Lai Jiangshan <jiangshanlai@gmail.com>]

On Tue, Dec 15, 2020 at 1:25 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Dec 14, 2020 at 11:54:49PM +0800, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@linux.alibaba.com>
> >
> > There might be other CPU online. The workers losing binding on its CPU
> > should have chance to work on those later onlined CPUs.
> >
> > Fixes: 06249738a41a ("workqueue: Manually break affinity on hotplug")
> > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> > ---
> >  kernel/workqueue.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index aba71ab359dd..1f5b8385c0cf 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -4909,8 +4909,9 @@ static void unbind_workers(int cpu)
> >
> >               raw_spin_unlock_irq(&pool->lock);
> >
> > +             /* don't rely on the scheduler to force break affinity for us. */
> >               for_each_pool_worker(worker, pool)
> > -                     WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_active_mask) < 0);
> > +                     WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0);
>
> Please explain this one.. it's not making sense. Also the Changelog
> doesn't seem remotely related to the actual change.

If the scheduler doesn't break affinity for us any more, I hope that
we can "emulate" previous behavior when the scheduler did breaks affinity
for us. The behavior is "changing the cpumask to cpu_possible_mask".

And there might be some other CPUs online later while the worker is
still running with the pending work items.  I hope the worker can also
use the later online CPUs as before.  If we use cpu_active_mask here,
we can't achieve this.   This is what the changelog said.  I don't know
which wording is better, I will combine both if this reason stands.


>
> Afaict this is actively wrong.
>
> Also, can you please not Cc me parts of a series? That's bloody
> annoying.


Sorry about it.  I was taught "once don't send the whole series to
someone" and very probably I missed the conditions about it.  I think
in this case, I should Cc you the whole series.  May I?

Thanks
Lai