On Fri 04-10-19 15:32:01, Konstantin Khlebnikov wrote: > > > On 04/10/2019 15.27, Michal Hocko wrote: > > 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. > > > > Caller might expect that pages added by him before are drained. > Exiting after mutex_trylock() will not guarantee that. > > For example POSIX_FADV_DONTNEED uses that.
OK, I was not aware of this case. Please make sure to document that in the changelog and a comment in the code wouldn't hurt either. It would certainly explain more thatn "Piggyback on drain done by somebody else.". Thanks! -- Michal Hocko SUSE Labs
