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.
