Re: [PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Nov 30, 2021 at 11:14:32AM +0100, Mike Galbraith wrote:
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index fb9584641ac7..1af12072f40e 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1021,6 +1021,39 @@ static void handle_write_error(struct address_space *mapping,
> >         unlock_page(page);
> >  }
> >  
> > +bool skip_throttle_noprogress(pg_data_t *pgdat)
> > +{
> > +       int reclaimable = 0, write_pending = 0;
> > +       int i;
> > +
> > +       /*
> > +        * If kswapd is disabled, reschedule if necessary but do not
> > +        * throttle as the system is likely near OOM.
> > +        */
> > +       if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
> > +               return true;
> > +
> > +       /*
> > +        * If there are a lot of dirty/writeback pages then do not
> > +        * throttle as throttling will occur when the pages cycle
> > +        * towards the end of the LRU if still under writeback.
> > +        */
> > +       for (i = 0; i < MAX_NR_ZONES; i++) {
> > +               struct zone *zone = pgdat->node_zones + i;
> > +
> > +               if (!populated_zone(zone))
> > +                       continue;
> > +
> > +               reclaimable += zone_reclaimable_pages(zone);
> > +               write_pending += zone_page_state_snapshot(zone,
> > +                                                 NR_ZONE_WRITE_PENDING);
> > +       }
> > +       if (2 * write_pending <= reclaimable)
> 
> That is always true here...
> 

Always true for you or always true in general?

The intent of the check is "are a majority of reclaimable pages
marked WRITE_PENDING?". It's similar to the check that existed prior
to 132b0d21d21f ("mm/page_alloc: remove the throttling logic from the
page allocator").

-- 
Mel Gorman
SUSE Labs