Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects

[xunlei <xlpang@linux.alibaba.com>]

On 2020/7/2 PM 7:59, Pekka Enberg wrote:
> On Thu, Jul 2, 2020 at 11:32 AM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>> The node list_lock in count_partial() spend long time iterating
>> in case of large amount of partial page lists, which can cause
>> thunder herd effect to the list_lock contention, e.g. it cause
>> business response-time jitters when accessing "/proc/slabinfo"
>> in our production environments.
> 
> Would you have any numbers to share to quantify this jitter? I have no

We have HSF RT(High-speed Service Framework Response-Time) monitors, the
RT figures fluctuated randomly, then we deployed a tool detecting "irq
off" and "preempt off" to dump the culprit's calltrace, capturing the
list_lock cost up to 100ms with irq off issued by "ss", this also caused
network timeouts.

> objections to this approach, but I think the original design
> deliberately made reading "/proc/slabinfo" more expensive to avoid
> atomic operations in the allocation/deallocation paths. It would be
> good to understand what is the gain of this approach before we switch
> to it. Maybe even run some slab-related benchmark (not sure if there's
> something better than hackbench these days) to see if the overhead of
> this approach shows up.

I thought that before, but most atomic operations are serialized by the
list_lock. Another possible way is to hold list_lock in __slab_free(),
then these two counters can be changed from atomic to long.

I also have no idea what's the standard SLUB benchmark for the
regression test, any specific suggestion?

> 
>> This patch introduces two counters to maintain the actual number
>> of partial objects dynamically instead of iterating the partial
>> page lists with list_lock held.
>>
>> New counters of kmem_cache_node are: pfree_objects, ptotal_objects.
>> The main operations are under list_lock in slow path, its performance
>> impact is minimal.
>>
>> Co-developed-by: Wen Yang <wenyang@linux.alibaba.com>
>> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
>> ---
>>  mm/slab.h |  2 ++
>>  mm/slub.c | 38 +++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 39 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/slab.h b/mm/slab.h
>> index 7e94700..5935749 100644
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -616,6 +616,8 @@ struct kmem_cache_node {
>>  #ifdef CONFIG_SLUB
>>         unsigned long nr_partial;
>>         struct list_head partial;
>> +       atomic_long_t pfree_objects; /* partial free objects */
>> +       atomic_long_t ptotal_objects; /* partial total objects */
> 
> You could rename these to "nr_partial_free_objs" and
> "nr_partial_total_objs" for readability.

Sounds good.

Thanks!

> 
> - Pekka
>