On Fri 04-10-19 05:10:17, Matthew Wilcox wrote: > On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote: > > This is very slow operation. There is no reason to do it again if somebody > > else already drained all per-cpu vectors after we waited for lock. > > + seq = raw_read_seqcount_latch(&seqcount); > > + > > mutex_lock(&lock); > > + > > + /* Piggyback on drain done by somebody else. */ > > + if (__read_seqcount_retry(&seqcount, seq)) > > + goto done; > > + > > + raw_write_seqcount_latch(&seqcount); > > + > > Do we really need the seqcount to do this? Wouldn't a mutex_trylock() > have the same effect?
Yeah, this makes sense. From correctness point of view it should be ok because no caller can expect that per-cpu pvecs are empty on return. This might have some runtime effects that some paths might retry more - e.g. offlining path drains pcp pvces before migrating the range away, if there are pages still waiting for a worker to drain them then the migration would fail and we would retry. But this not a correctness issue. -- Michal Hocko SUSE Labs
