linux-kernel - Re: [PATCH v8 04/23] slab: add sheaf support for batching kfree

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Message-ID: <CAJuCfpG4JYPE0YbzcLCZbtsqkqCrzaRENkBunpvzLC_B5q2vCA@mail.gmail.com>
Date: Fri, 26 Sep 2025 08:41:45 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Uladzislau Rezki <urezki@...il.com>, 
	Sidhartha Kumar <sidhartha.kumar@...cle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org, 
	maple-tree@...ts.infradead.org, "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v8 04/23] slab: add sheaf support for batching kfree_rcu() operations

On Fri, Sep 26, 2025 at 3:08 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 9/25/25 06:35, Suren Baghdasaryan wrote:
> > On Thu, Sep 18, 2025 at 1:09 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >> Expected advantages:
> >> - batching the kfree_rcu() operations, that could eventually replace the
> >>   existing batching
> >> - sheaves can be reused for allocations via barn instead of being
> >>   flushed to slabs, which is more efficient
> >>   - this includes cases where only some cpus are allowed to process rcu
> >>     callbacks (Android)
> >
> > nit: I would say it's more CONFIG_RCU_NOCB_CPU related. Android is
> > just an instance of that.
>
> OK changed that.
>
> Changes due to your other suggestions:
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 8220ce095970..fec0cdc7ef37 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3945,15 +3945,12 @@ void flush_all_rcu_sheaves(void)
>                          */
>
>                         INIT_WORK(&sfw->work, flush_rcu_sheaf);
> -                       sfw->skip = false;
>                         sfw->s = s;
>                         queue_work_on(cpu, flushwq, &sfw->work);
>                 }
>
>                 for_each_online_cpu(cpu) {
>                         sfw = &per_cpu(slub_flush, cpu);
> -                       if (sfw->skip)
> -                               continue;
>                         flush_work(&sfw->work);
>                 }
>
> @@ -5643,6 +5640,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
>
>         rcu_sheaf = pcs->rcu_free;
>
> +       /*
> +        * Since we flush immediately when size reaches capacity, we never reach
> +        * this with size already at capacity, so no OOB write is possible.
> +        */

Perfect!

Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

>         rcu_sheaf->objects[rcu_sheaf->size++] = obj;
>
>         if (likely(rcu_sheaf->size < s->sheaf_capacity))
>