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 will get a potentially slightly outdated "new scan count" value, but 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. > > > > > 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()? 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. Thanks!
