linux-kernel - Re: [PATCH RFC v2 08/20] slab: add optimized sheaf refill from partial list

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Message-ID: <bb58c778-be6b-445e-a331-ddaf04f97f0e@suse.cz>
Date: Fri, 16 Jan 2026 08:32:00 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Hao Li <hao.li@...ux.dev>
Cc: Harry Yoo <harry.yoo@...cle.com>, Petr Tesarik <ptesarik@...e.com>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Suren Baghdasaryan <surenb@...gle.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
 bpf@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH RFC v2 08/20] slab: add optimized sheaf refill from
 partial list

On 1/16/26 07:27, Hao Li wrote:
> On Thu, Jan 15, 2026 at 03:25:59PM +0100, Vlastimil Babka wrote:
>> On 1/12/26 16:17, Vlastimil Babka wrote:
>> > At this point we have sheaves enabled for all caches, but their refill
>> > is done via __kmem_cache_alloc_bulk() which relies on cpu (partial)
>> > slabs - now a redundant caching layer that we are about to remove.
>> > 
>> > The refill will thus be done from slabs on the node partial list.
>> > Introduce new functions that can do that in an optimized way as it's
>> > easier than modifying the __kmem_cache_alloc_bulk() call chain.
>> > 
>> > Extend struct partial_context so it can return a list of slabs from the
>> > partial list with the sum of free objects in them within the requested
>> > min and max.
>> > 
>> > Introduce get_partial_node_bulk() that removes the slabs from freelist
>> > and returns them in the list.
>> > 
>> > Introduce get_freelist_nofreeze() which grabs the freelist without
>> > freezing the slab.
>> > 
>> > Introduce alloc_from_new_slab() which can allocate multiple objects from
>> > a newly allocated slab where we don't need to synchronize with freeing.
>> > In some aspects it's similar to alloc_single_from_new_slab() but assumes
>> > the cache is a non-debug one so it can avoid some actions.
>> > 
>> > Introduce __refill_objects() that uses the functions above to fill an
>> > array of objects. It has to handle the possibility that the slabs will
>> > contain more objects that were requested, due to concurrent freeing of
>> > objects to those slabs. When no more slabs on partial lists are
>> > available, it will allocate new slabs. It is intended to be only used
>> > in context where spinning is allowed, so add a WARN_ON_ONCE check there.
>> > 
>> > Finally, switch refill_sheaf() to use __refill_objects(). Sheaves are
>> > only refilled from contexts that allow spinning, or even blocking.
>> > 
>> > Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> 
>> ...
>> 
>> > +static unsigned int alloc_from_new_slab(struct kmem_cache *s, struct slab *slab,
>> > +		void **p, unsigned int count, bool allow_spin)
>> > +{
>> > +	unsigned int allocated = 0;
>> > +	struct kmem_cache_node *n;
>> > +	unsigned long flags;
>> > +	void *object;
>> > +
>> > +	if (!allow_spin && (slab->objects - slab->inuse) > count) {
>> > +
>> > +		n = get_node(s, slab_nid(slab));
>> > +
>> > +		if (!spin_trylock_irqsave(&n->list_lock, flags)) {
>> > +			/* Unlucky, discard newly allocated slab */
>> > +			defer_deactivate_slab(slab, NULL);
>> 
>> This actually does dec_slabs_node() only with slab->frozen which we don't set.
> 
> Hi, I think I follow the intent, but I got a little tripped up here: patch 08
> (current patch) seems to assume "slab->frozen = 1" is already gone. That's true
> after the whole series, but the removal only happens in patch 09.
> 
> Would it make sense to avoid relying on that assumption when looking at patch 08
> in isolation?

Hm I did think it's fine. alloc_from_new_slab() introduced here is only used
from __refill_objects() and that one doesn't set slab->frozen = 1 on the new
slab?

Then patch 09 switches ___slab_alloc() to alloc_from_new_slab() and at the
same time also stops setting slab->frozen = 1 so it should be also fine.

And then 12/20 slab: remove defer_deactivate_slab() removes the frozen = 1
treatment as nobody uses it anymore.

If there's some mistake in the above, please tell!

Thanks.