linux-kernel - Re: [RFC 11/26] mm, slub: move disabling irqs closer to get_partial() in ___slab

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Message-ID: <CAG48ez0mXxqLdSse6OpAMLNxSMc7nNor8PXk=kagPppGAXAXgA@mail.gmail.com>
Date:   Tue, 25 May 2021 18:00:23 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC 11/26] mm, slub: move disabling irqs closer to get_partial()
 in ___slab_alloc()

On Tue, May 25, 2021 at 1:40 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> Continue reducing the irq disabled scope. Check for per-cpu partial slabs with
> first with irqs enabled and then recheck with irqs disabled before grabbing
> the slab page. Mostly preparatory for the following patches.
[...]
> diff --git a/mm/slub.c b/mm/slub.c
[...]
>         if (slub_percpu_partial(c)) {
> +               local_irq_save(flags);
> +               if (unlikely(c->page)) {
> +                       local_irq_restore(flags);
> +                       goto reread_page;
> +               }
> +               if (unlikely(!slub_percpu_partial(c))) /* stolen by IRQ? */
> +                       goto new_objects;

nit: I think this comment is wrong by the end of the patch series,
since at that point, in RT configurations, it could also be stolen by
another task, if I understand correctly what migrate_disable() means?

Similarly the comment above ___slab_alloc() still talks about
disabling preemption for bulk allocation.