Re: [PATCH] slub: avoid list_lock contention from __refill_objects_any()

[Vlastimil Babka]

On 1/29/26 10:30, Harry Yoo wrote:
> On Thu, Jan 29, 2026 at 05:21:21PM +0800, Hao Li wrote:
>> On Thu, Jan 29, 2026 at 10:07:57AM +0100, Vlastimil Babka wrote:
>> > Kernel test robot has reported a regression in the patch "slab: refill
>> > sheaves from all nodes". When taken in isolation like this, there is
>> > indeed a tradeoff - we prefer to use remote objects prior to allocating
>> > new local slabs. It is replicating a behavior that existed before
>> > sheaves for replenishing cpu (partial) slabs - now called
>> > get_from_any_partial() to allocate a single object.
>> > 
>> > So the possibility of allocating remote objects is intended even if
>> > remote accesses are then slower. But the profiles in the report also
>> > suggested a contention on the list_lock spinlock. And that's something
>> > we can try to avoid without much tradeoff - if someone else has the
>> > spin_lock, it's more likely they are allocating from the node than
>> > freeing to it, so we can skip it even if it means allocating a new local
>> > slab - contributing to that lock's contention isn't worth it. It should
>> > not result in partial slabs accumulating on the remote node.
>> > 
>> > Thus add an allow_spin parameter to __refill_objects_node() and
>> > get_partial_node_bulk() to make the attempts from __refill_objects_any()
>> > use only a trylock.
>> > 
>> > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
>> > Link: https://lore.kernel.org/oe-lkp/202601132136.77efd6d7-lkp@xxxxxxxxx
>> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> 
>> In my testing, this patch improved performance by:
>> 
>> will-it-scale.64.processes +14.2%
>> will-it-scale.128.processes +9.6%
>> will-it-scale.192.processes +10.8%
>> will-it-scale.per_process_ops +11.6%
>>
>> Tested-by: Hao Li <hao.li@xxxxxxxxx>
> 
> I wonder if using spin_is_contended() or spin_is_locked()
> would be better than trylock by avoiding an atomic operation?

I checked and found that spin_trylock() itself implements a non-atomic check
before the atomic. So adding a spin_is_locked() would only help the caller
bail out a bit faster, but this is not a fastpath. It wouldn't help the
cache coherency traffic, AFAIU.