Re: [PATCH] [Cleanup] memcg: export memory cgroup's swappiness v2

KAMEZAWA Hiroyuki
2011-07-01T01:23:33Z

On Thu, 30 Jun 2011 18:06:53 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 1 Jul 2011 09:20:59 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
> > On Fri, 1 Jul 2011 08:50:13 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >
> > > On Thu, 30 Jun 2011 13:01:34 -0700
> > > Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > > Ok, I'll check it. Maybe I miss !CONFIG_SWAP...
> > >
> >
> > v4 here. Thank you for pointing out. I could think of several ways but
> > maybe this one is good because using vm_swappines with !CONFIG_SWAP seems
> > to be a bug.
>
> No, it isn't a bug - swappiness also controls the kernel's eagerness to
> unmap and reclaim mmapped pagecache.
>

Oh, really ? I didn't understand that.

> > tested with allyesconfig/allnoconfig.
>
> Did it break the above?
>

Hmm. If !CONFIG_SWAP, get_scan_count() will see !nr_swap_pages and
set scan ratio as
file: 100%
anon: 0%

> > +#ifdef CONFIG_SWAP
> > +static int vmscan_swappiness(struct scan_control *sc)
> > +{
> > + if (scanning_global_lru(sc))
> > + return vm_swappiness;
>
> Well that's a bit ugly - it assumes that all callers set
> scan_control.swappiness to vm_swappiness then never change it. That
> may be true in the current code.
>
> Ho hum, I guess that's a simplification we can make.
>

We don't calculate kernel internal swappiness and just use vm_swappines
which the user specified. So, I thought it should not be in scan_control.

> > + return mem_cgroup_swappiness(sc->mem_cgroup);
> > +}
> > +#else
> > +static int vmscan_swappiness(struct scan_control *sc)
> > +{
> > + /* Now, this function is never called with !CONFIG_SWAP */
> > + BUG();
> > + return 0;
> > +}
> > +#endif
> >
> > ...
> >
> > @@ -1789,8 +1804,8 @@ static void get_scan_count(struct zone *zone, struct scan_control *sc,
> > * With swappiness at 100, anonymous and file have the same priority.
> > * This scanning priority is essentially the inverse of IO cost.
> > */
> > - anon_prio = sc->swappiness;
> > - file_prio = 200 - sc->swappiness;
> > + anon_prio = vmscan_swappiness(sc);
> > + file_prio = 200 - vmscan_swappiness(sc);
>
> hah, this should go BUG if CONFIG_SWAP=n. But it won't, because we
> broke get_scan_count(). It fails to apply vm_swappiness to file-backed
> pages if there's no available swap, which is daft.
>
> I think this happened in 76a33fc380c9a ("vmscan: prevent
> get_scan_ratio() rounding errors") which claims "this patch doesn't
> really change logics, but just increase precision".
>

Hmm, IIUC.
- the controller of unmapping file cache is now sc->may_unmap
- may_unmap is always set 1 unless zone_reclaim_mode.
- vm_swappiness doesn't affect it now.
- file LRU contains both mapped and unmapped pages in the same manner
- get_scan_count() cannot help decisiion of "map or unmap"
- Active/Inactive scan ratio is now determined by reclaim_stat.

Hmm, swappiness should affect active/inactive scan ratio ?

Thanks,
-Kame