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

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Message-ID: <80f8bdf8-ce50-4682-9468-9f38e0817e89@suse.cz>
Date: Fri, 26 Sep 2025 12:08:26 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
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 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.
+        */
        rcu_sheaf->objects[rcu_sheaf->size++] = obj;

        if (likely(rcu_sheaf->size < s->sheaf_capacity))