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 will get a potentially slightly outdated "new scan count" value, but 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? > > Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx> > --- > 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()? -- Thanks, David / dhildenb
