Re: [PATCH] mm: do not call add_nr_deferred() with zero deferred

David Hildenbrand
2022-04-19T16:57:22Z

On 19.04.22 18:42, Roman Gushchin wrote:
> On Tue, Apr 19, 2022 at 02:56:06PM +0200, David Hildenbrand wrote:
>> On 16.04.22 02:41, Roman Gushchin wrote:
>>> add_nr_deferred() is often called with next_deferred equal to 0.
>>> For instance, it's happening under low memory pressure for any
>>> shrinkers with a low number of cached objects. A corresponding trace
>>> looks like:
>>> <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \
>>> super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \
>>> unused scan count 0 new scan count 0 total_scan 0 \
>>> last shrinker return val 0
>>>
>>> <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \
>>> super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \
>>> unused scan count 0 new scan count 0 total_scan 0 \
>>> last shrinker return val 0
>>>
>>> <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \
>>> super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \
>>> scan count 0 new scan count 0 total_scan 0 \
>>> last shrinker return val 0
>>>
>>> This lead to unnecessary checks and atomic operations, which can be
>>> avoided by checking next_deferred for not being zero before calling
>>> add_nr_deferred(). In this case the mm_shrink_slab_end trace point
>>> will get a potentially slightly outdated "new scan count" value, but
>>> it's totally fine.
>>
>> Sufficient improvement to justify added complexity for anybody reading
>> that code?
>
> I don't have any numbers and really doubt the difference is significant,
> however the added complexity is also small: one "if" statement.
> Anyway, if you feel strongly against this change, I'm fine with dropping it.
>

No strong opinion, naturally, more conditions make the code harder to
read -- that's why I'm asking.

>>
>>>
>>> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
>>> ---
>>> mm/vmscan.c | 5 ++++-
>>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index d4a7d2bd276d..19d3d4fa1aad 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>>> * move the unused scan count back into the shrinker in a
>>> * manner that handles concurrent updates.
>>> */
>>> - new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
>>> + if (next_deferred)
>>> + new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
>>> + else
>>> + new_nr = nr;
>>>
>>> trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan);
>>> return freed;
>>
>> And if we still want to do this optimization, why not put it into
>> add_nr_deferred()?
>
> Because of the semantics of add_nr_deferred(), which returns the deferred value.
> It's not used for anything except tracing, so maybe it's a place for another
> change.

Skimming over the code I somehow missed that add_nr_deferred() doesn't
have "nr" naturally available.

LGTM

Acked-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb