Re: [PATCH 6/9] mm: vmscan: use per memcg nr_deferred of shrinker
From: Yang Shi
Date: Wed Dec 09 2020 - 12:33:34 EST
On Wed, Dec 9, 2020 at 7:42 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>
> On 08.12.2020 20:13, Yang Shi wrote:
> > On Thu, Dec 3, 2020 at 3:40 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> >>
> >> On 02.12.2020 21:27, Yang Shi wrote:
> >>> Use per memcg's nr_deferred for memcg aware shrinkers. The shrinker's nr_deferred
> >>> will be used in the following cases:
> >>> 1. Non memcg aware shrinkers
> >>> 2. !CONFIG_MEMCG
> >>> 3. memcg is disabled by boot parameter
> >>>
> >>> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> >>> ---
> >>> mm/vmscan.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++----
> >>> 1 file changed, 82 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >>> index cba0bc8d4661..d569fdcaba79 100644
> >>> --- a/mm/vmscan.c
> >>> +++ b/mm/vmscan.c
> >>> @@ -203,6 +203,12 @@ static DECLARE_RWSEM(shrinker_rwsem);
> >>> static DEFINE_IDR(shrinker_idr);
> >>> static int shrinker_nr_max;
> >>>
> >>> +static inline bool is_deferred_memcg_aware(struct shrinker *shrinker)
> >>> +{
> >>> + return (shrinker->flags & SHRINKER_MEMCG_AWARE) &&
> >>> + !mem_cgroup_disabled();
> >>> +}
> >>> +
> >>> static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >>> {
> >>> int id, ret = -ENOMEM;
> >>> @@ -271,7 +277,58 @@ static bool writeback_throttling_sane(struct scan_control *sc)
> >>> #endif
> >>> return false;
> >>> }
> >>> +
> >>> +static inline long count_nr_deferred(struct shrinker *shrinker,
> >>> + struct shrink_control *sc)
> >>> +{
> >>> + bool per_memcg_deferred = is_deferred_memcg_aware(shrinker) && sc->memcg;
> >>> + struct memcg_shrinker_deferred *deferred;
> >>> + struct mem_cgroup *memcg = sc->memcg;
> >>> + int nid = sc->nid;
> >>> + int id = shrinker->id;
> >>> + long nr;
> >>> +
> >>> + if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> >>> + nid = 0;
> >>> +
> >>> + if (per_memcg_deferred) {
> >>> + deferred = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_deferred,
> >>> + true);
> >>
> >> My comment is about both 5/9 and 6/9 patches.
> >
> > Sorry for the late reply, I don't know why Gmail filtered this out to spam.
> >
> >>
> >> shrink_slab_memcg() races with mem_cgroup_css_online(). A visibility of CSS_ONLINE flag
> >> in shrink_slab_memcg()->mem_cgroup_online() does not guarantee that you will see
> >> memcg->nodeinfo[nid]->shrinker_deferred != NULL in count_nr_deferred(). This may occur
> >> because of processor reordering on !x86 (there is no a common lock or memory barriers).
> >>
> >> Regarding to shrinker_map this is not a problem due to map check in shrink_slab_memcg().
> >> The map can't be NULL there.
> >>
> >> Regarding to shrinker_deferred you should prove either this is not a problem too,
> >> or to add proper synchronization (maybe, based on barriers) or to add some similar check
> >> (maybe, in shrink_slab_memcg() too).
> >
> > It seems shrink_slab_memcg() might see shrinker_deferred as NULL
> > either due to the same reason. I don't think there is a guarantee it
> > won't happen.
> >
> > We just need guarantee CSS_ONLINE is seen after shrinker_maps and
> > shrinker_deferred are allocated, so I'm supposed barriers before
> > "css->flags |= CSS_ONLINE" should work.
> >
> > So the below patch may be ok:
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index df128cab900f..9f7fb0450d69 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5539,6 +5539,12 @@ static int mem_cgroup_css_online(struct
> > cgroup_subsys_state *css)
> > return -ENOMEM;
> > }
> >
> >
> > + /*
> > + * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees
> > shirnker_maps
> > + * and shrinker_deferred before CSS_ONLINE.
> > + */
> > + smp_mb();
> > +
> > /* Online state pins memcg ID, memcg ID pins CSS */
> > refcount_set(&memcg->id.ref, 1);
> > css_get(css);
>
> smp barriers synchronize data access from different cpus. They should go in a pair.
> In case of you add the smp barrier into mem_cgroup_css_online(), we should also
> add one more smp barrier in another place, which we want to synchonize with this.
> Also, every place should contain a comment referring to its pair: "Pairs with...".
Thanks, I think you are correct. Looked into it further, it seems the
race pattern looks like:
CPU A CPU B
store shrinker_maps pointer load CSS_ONLINE
store CSS_ONLINE load shrinker_maps pointer
By checking the memory-barriers document, it seems we need write
barrier/read barrier pair as below:
CPU A CPU B
store shrinker_maps pointer load CSS_ONLINE
<write barrier> <read barrier>
store CSS_ONLINE load shrinker_maps pointer
So, the patch should look like:
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index df128cab900f..489c0a84f82b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5539,6 +5539,13 @@ static int mem_cgroup_css_online(struct
cgroup_subsys_state *css)
return -ENOMEM;
}
+ /*
+ * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees
shirnker_maps
+ * and shrinker_deferred before CSS_ONLINE. It pairs with the
read barrier
+ * in shrink_slab_memcg().
+ */
+ smp_wmb();
+
/* Online state pins memcg ID, memcg ID pins CSS */
refcount_set(&memcg->id.ref, 1);
css_get(css);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9d2a6485e982..fc9bda576d98 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -603,13 +603,15 @@ static unsigned long shrink_slab_memcg(gfp_t
gfp_mask, int nid,
if (!mem_cgroup_online(memcg))
return 0;
+ /* Pairs with write barrier in mem_cgroup_css_online */
+ smp_rmb();
+
if (!down_read_trylock(&shrinker_rwsem))
return 0;
+ /* Once memcg is online it can't be NULL */
map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,
true);
- if (unlikely(!map))
- goto unlock;
for_each_set_bit(i, map->map, shrinker_nr_max) {
struct shrink_control sc = {
Does this seem correct?
>
> Kirill
>