On Wed, Feb 22, 2017 at 04:00:36PM +0900, Minchan Kim wrote: > > There are also more allocation stalls. One of the largest impacts was due > > to pages written back from kswapd context rising from 0 pages to 4516642 > > pages during the hour the workload ran for. By and large, the patch has very > > bad behaviour but easily missed as the impact on a UMA machine is > > negligible. > > > > This patch is included with the data in case a bisection leads to this area. > > This patch is also a pre-requisite for the rest of the series. > > > > Signed-off-by: Shantanu Goel <[email protected]> > > Signed-off-by: Mel Gorman <[email protected]> > > Hmm, I don't understand why we should bind wakeup_kcompactd to kswapd's > short sleep point where every eligible zones are balanced. > What's the correlation between them? >
If kswapd is ready for a short sleep, eligible zones are balanced for order-0 but not necessarily the originally requested order if kswapd gave up reclaiming as compaction was ready to start. As kswapd is ready to sleep for a short period, it's a suitable time for kcompactd to decide if it should start working or not. There is no need for kswapd to be aware of kcompactd's wakeup criteria. > Can't we wake up kcompactd once we found a zone has enough free pages > above high watermark like this? > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 26c3b405ef34..f4f0ad0e9ede 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -3346,13 +3346,6 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int > alloc_order, int reclaim_o > * that pages and compaction may succeed so reset the cache. > */ > reset_isolation_suitable(pgdat); > - > - /* > - * We have freed the memory, now we should compact it to make > - * allocation of the requested order possible. > - */ > - wakeup_kcompactd(pgdat, alloc_order, classzone_idx); > - > remaining = schedule_timeout(HZ/10); > > /* > @@ -3451,6 +3444,14 @@ static int kswapd(void *p) > bool ret; > > kswapd_try_sleep: > + /* > + * We have freed the memory, now we should compact it to make > + * allocation of the requested order possible. > + */ > + if (alloc_order > 0 && zone_balanced(zone, reclaim_order, > + classzone_idx)) > + wakeup_kcompactd(pgdat, alloc_order, classzone_idx); > + > kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order, > classzone_idx); That's functionally very similar to what happens already. wakeup_kcompactd checks the order and does not wake for order-0. It also makes its own decisions that include zone_balanced on whether it is safe to wakeup. I doubt there would be any measurable difference from a patch like this and to my mind at least, it does not improve the readability or flow of the code. -- Mel Gorman SUSE Labs
