Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210524233946.20352-1-vbabka@suse.cz> <20210524233946.20352-12-vbabka@suse.cz>
In-Reply-To: <20210524233946.20352-12-vbabka@suse.cz>
From:   Jann Horn <jannh@google.com>
Date:   Tue, 25 May 2021 18:00:23 +0200
Message-ID: <CAG48ez0mXxqLdSse6OpAMLNxSMc7nNor8PXk=kagPppGAXAXgA@mail.gmail.com>
Subject: Re: [RFC 11/26] mm, slub: move disabling irqs closer to get_partial()
 in ___slab_alloc()
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, May 25, 2021 at 1:40 AM Vlastimil Babka <vbabka@suse.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.