> On Fri 30-10-15 09:36:26, Michal Hocko wrote:
> > On Fri 30-10-15 12:10:15, Hillf Danton wrote:
> > [...]
> > > > + for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx, ac->nodemask) {
> > > > + unsigned long free = zone_page_state(zone, NR_FREE_PAGES);
> > > > + unsigned long reclaimable;
> > > > + unsigned long target;
> > > > +
> > > > + reclaimable = zone_reclaimable_pages(zone) +
> > > > + zone_page_state(zone, NR_ISOLATED_FILE) +
> > > > + zone_page_state(zone, NR_ISOLATED_ANON);
> > > > + target = reclaimable;
> > > > + target -= stall_backoff * (1 + target/MAX_STALL_BACKOFF);
> > >
> > > target = reclaimable - stall_backoff * (1 + target/MAX_STALL_BACKOFF);
> > > = reclaimable - stall_backoff - stall_backoff * (target/MAX_STALL_BACKOFF);
> > >
> > > then the first stall_backoff looks unreasonable.
> >
> > First stall_backoff is off by 1 but that shouldn't make any difference.
> >
> > > I guess you mean
> > > target = reclaimable - target * (stall_backoff/MAX_STALL_BACKOFF);
> > > = reclaimable - stall_back * (target/MAX_STALL_BACKOFF);
> >
> > No the reason I used the bias is to converge for MAX_STALL_BACKOFF. If
> > you have target which is not divisible by MAX_STALL_BACKOFF then the
> > rounding would get target > 0 and so we wouldn't converge. With the +1
> > you underflow which is MAX_STALL_BACKOFF in maximum which should be
> > fixed up by the free memory. Maybe a check for free < MAX_STALL_BACKOFF
> > would be good but I didn't get that far with this.
>
> I've ended up with the following after all. It uses ceiling for the
> division this should be underflow safe albeit less readable (at least
> for me).
Looks good, thanks.
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
> ---
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0dc1ca9b1219..c9a4e62f234e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3176,7 +3176,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> zone_page_state(zone, NR_ISOLATED_FILE) +
> zone_page_state(zone, NR_ISOLATED_ANON);
> target = reclaimable;
> - target -= stall_backoff * (1 + target/MAX_STALL_BACKOFF);
> + target -= (stall_backoff * target + MAX_STALL_BACKOFF - 1) / MAX_STALL_BACKOFF;
> target += free;
>
> /*
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bc14217acd47..0b3ec972ec7a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2672,7 +2672,6 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> int initial_priority = sc->priority;
> unsigned long total_scanned = 0;
> unsigned long writeback_threshold;
> - bool zones_reclaimable;
> retry:
> delayacct_freepages_start();
>
> @@ -2683,7 +2682,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
> sc->priority);
> sc->nr_scanned = 0;
> - zones_reclaimable = shrink_zones(zonelist, sc);
> + shrink_zones(zonelist, sc);
>
> total_scanned += sc->nr_scanned;
> if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> --
> Michal Hocko
> SUSE Labs
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