linux-kernel - Re: [External] Re: [PATCH] mm: slub: replace local_irq_save with local_irq

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Message-ID: <CAMZfGtXPEexiKSMcViKuFnRoYjH7ajjXGLRS8Q-H-pTmyHsCbA@mail.gmail.com>
Date:   Thu, 10 Jun 2021 12:14:18 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH] mm: slub: replace local_irq_save with local_irq_disable

On Mon, Jun 7, 2021 at 8:32 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 6/6/21 6:17 AM, Muchun Song wrote:
> > The caller of slub_cpu_dead cannot be irq disabled (because slab_mutex is
> > holding during the processing), there is no need to use irq_save. Just use
> > irq_disable directly.
>
> Well, we shouldn't need to disable irq at all. We are cleaning up for a dead
> cpu, so there's nobody else accesing the data. irq save/disable will protect
> only the local cpu's data, not of the cpu we are flushing. But we can't simply
> remove the irq disable/enable because there are some nested
> VM_BUG_ON(!irqs_disabled()) under __flush_cpu_slab(). We basically only disable
> irqs here to avoid those from triggering.
>
> My series [1] addresses this completely (among other things), but it's early
> stage RFC (v2 should be soon). Your patch is not wrong, but also not urgent or
> perf critical. So with that context I'll leave the decision to others :)

Got it.

>
> [1] https://lore.kernel.org/lkml/20210524233946.20352-1-vbabka@suse.cz/

Good series. Reducing irq disabling is nice.

Thanks.

>
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > ---
> >  mm/slub.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index ee51857d8e9b..fbf592ef14ff 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2529,13 +2529,12 @@ static void flush_all(struct kmem_cache *s)
> >  static int slub_cpu_dead(unsigned int cpu)
> >  {
> >       struct kmem_cache *s;
> > -     unsigned long flags;
> >
> >       mutex_lock(&slab_mutex);
> >       list_for_each_entry(s, &slab_caches, list) {
> > -             local_irq_save(flags);
> > +             local_irq_disable();
> >               __flush_cpu_slab(s, cpu);
> > -             local_irq_restore(flags);
> > +             local_irq_enable();
> >       }
> >       mutex_unlock(&slab_mutex);
> >       return 0;
> >
>