Re: [PATCH] mm: vmscan: consistent update to pgsteal and pgscan

[Shakeel Butt]

On Fri, May 8, 2020 at 3:34 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> On Fri, May 8, 2020 at 4:49 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> >
> > One way to measure the efficiency of memory reclaim is to look at the
> > ratio (pgscan+pfrefill)/pgsteal. However at the moment these stats are
> > not updated consistently at the system level and the ratio of these are
> > not very meaningful. The pgsteal and pgscan are updated for only global
> > reclaim while pgrefill gets updated for global as well as cgroup
> > reclaim.
> >
>
> Hi Shakeel,
>
> We always use pgscan and pgsteal for monitoring the system level
> memory pressure, for example, by using sysstat(sar) or some other
> monitor tools.

Don't you need pgrefill in addition to pgscan and pgsteal to get the
full picture of the reclaim activity?

> But with this change, these two counters include the memcg pressure as
> well. It is not easy to know whether the pgscan and pgsteal are caused
> by system level pressure or only some specific memcgs reaching their
> memory limit.
>
> How about adding  cgroup_reclaim() to pgrefill as well ?
>

I am looking for all the reclaim activity on the system. Adding
!cgroup_reclaim to pgrefill will skip the cgroup reclaim activity.
Maybe adding pgsteal_cgroup and pgscan_cgroup would be better.

> > Please note that this difference is only for system level vmstats. The
> > cgroup stats returned by memory.stat are actually consistent. The
> > cgroup's pgsteal contains number of reclaimed pages for global as well
> > as cgroup reclaim. So, one way to get the system level stats is to get
> > these stats from root's memory.stat but root does not expose that
> > interface. Also for !CONFIG_MEMCG machines /proc/vmstat is the only way
> > to get these stats. So, make these stats consistent.
> >
> > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > ---
> >  mm/vmscan.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index cc555903a332..51f7d1efc912 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1943,8 +1943,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> >         reclaim_stat->recent_scanned[file] += nr_taken;
> >
> >         item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;
> > -       if (!cgroup_reclaim(sc))
> > -               __count_vm_events(item, nr_scanned);
> > +       __count_vm_events(item, nr_scanned);
> >         __count_memcg_events(lruvec_memcg(lruvec), item, nr_scanned);
> >         spin_unlock_irq(&pgdat->lru_lock);
> >
> > @@ -1957,8 +1956,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> >         spin_lock_irq(&pgdat->lru_lock);
> >
> >         item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;
> > -       if (!cgroup_reclaim(sc))
> > -               __count_vm_events(item, nr_reclaimed);
> > +       __count_vm_events(item, nr_reclaimed);
> >         __count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
> >         reclaim_stat->recent_rotated[0] += stat.nr_activate[0];
> >         reclaim_stat->recent_rotated[1] += stat.nr_activate[1];
> > --
> > 2.26.2.526.g744177e7f7-goog
> >
> >
>
>
> --
> Thanks
> Yafang