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From:   Nhat Pham <nphamcs@gmail.com>
Date:   Wed, 6 Dec 2023 08:56:43 -0800
Message-ID: <CAKEwX=M8YThH8qOdHt5TV1E4PCiw2FSv7815O3fhqXNVMt5ezg@mail.gmail.com>
Subject: Re: [PATCH v8 6/6] zswap: shrinks zswap pool based on memory pressure
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Chengming Zhou <chengming.zhou@linux.dev>,
        akpm@linux-foundation.org, hannes@cmpxchg.org,
        cerasuolodomenico@gmail.com, sjenning@redhat.com,
        ddstreet@ieee.org, vitaly.wool@konsulko.com, mhocko@kernel.org,
        roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, chrisl@kernel.org, linux-mm@kvack.org,
        kernel-team@meta.com, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org, shuah@kernel.org
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On Tue, Dec 5, 2023 at 10:00=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> [..]
> > > @@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get=
(struct rb_root *root,
> > >       return entry;
> > >  }
> > >
> > > +/*********************************
> > > +* shrinker functions
> > > +**********************************/
> > > +static enum lru_status shrink_memcg_cb(struct list_head *item, struc=
t list_lru_one *l,
> > > +                                    spinlock_t *lock, void *arg);
> > > +
> > > +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> > > +             struct shrink_control *sc)
> > > +{
> > > +     struct lruvec *lruvec =3D mem_cgroup_lruvec(sc->memcg, NODE_DAT=
A(sc->nid));
> > > +     unsigned long shrink_ret, nr_protected, lru_size;
> > > +     struct zswap_pool *pool =3D shrinker->private_data;
> > > +     bool encountered_page_in_swapcache =3D false;
> > > +
> > > +     nr_protected =3D
> > > +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_p=
rotected);
> > > +     lru_size =3D list_lru_shrink_count(&pool->list_lru, sc);
> > > +
> > > +     /*
> > > +      * Abort if the shrinker is disabled or if we are shrinking int=
o the
> > > +      * protected region.
> > > +      *
> > > +      * This short-circuiting is necessary because if we have too ma=
ny multiple
> > > +      * concurrent reclaimers getting the freeable zswap object coun=
ts at the
> > > +      * same time (before any of them made reasonable progress), the=
 total
> > > +      * number of reclaimed objects might be more than the number of=
 unprotected
> > > +      * objects (i.e the reclaimers will reclaim into the protected =
area of the
> > > +      * zswap LRU).
> > > +      */
> > > +     if (!zswap_shrinker_enabled || nr_protected >=3D lru_size - sc-=
>nr_to_scan) {
> > > +             sc->nr_scanned =3D 0;
> > > +             return SHRINK_STOP;
> > > +     }
> > > +
> > > +     shrink_ret =3D list_lru_shrink_walk(&pool->list_lru, sc, &shrin=
k_memcg_cb,
> > > +             &encountered_page_in_swapcache);
> > > +
> > > +     if (encountered_page_in_swapcache)
> > > +             return SHRINK_STOP;
> > > +
> > > +     return shrink_ret ? shrink_ret : SHRINK_STOP;
> > > +}
> > > +
> > > +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> > > +             struct shrink_control *sc)
> > > +{
> > > +     struct zswap_pool *pool =3D shrinker->private_data;
> > > +     struct mem_cgroup *memcg =3D sc->memcg;
> > > +     struct lruvec *lruvec =3D mem_cgroup_lruvec(memcg, NODE_DATA(sc=
->nid));
> > > +     unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> > > +
> > > +#ifdef CONFIG_MEMCG_KMEM
> > > +     cgroup_rstat_flush(memcg->css.cgroup);
> > > +     nr_backing =3D memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_S=
HIFT;
> > > +     nr_stored =3D memcg_page_state(memcg, MEMCG_ZSWAPPED);
> > > +#else
> > > +     /* use pool stats instead of memcg stats */
> > > +     nr_backing =3D get_zswap_pool_size(pool) >> PAGE_SHIFT;
> > > +     nr_stored =3D atomic_read(&pool->nr_stored);
> > > +#endif
> > > +
> > > +     if (!zswap_shrinker_enabled || !nr_stored)
> > When I tested with this series, with !zswap_shrinker_enabled in the def=
ault case,
> > I found the performance is much worse than that without this patch.
> >
> > Testcase: memory.max=3D2G, zswap enabled, kernel build -j32 in a tmpfs =
directory.
> >
> > The reason seems the above cgroup_rstat_flush(), caused much rstat lock=
 contention
> > to the zswap_store() path. And if I put the "zswap_shrinker_enabled" ch=
eck above
> > the cgroup_rstat_flush(), the performance become much better.
> >
> > Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_=
flush()?
>
> Yes, we should do nothing if !zswap_shrinker_enabled. We should also
> use mem_cgroup_flush_stats() here like other places unless accuracy is
> crucial, which I doubt given that reclaim uses
> mem_cgroup_flush_stats().

Ah, good points on both suggestions. We should not do extra work for
non-user. And, this is a best-effort approximation of the memory
saving factor, so as long as it is not *too* far off I think it's
acceptable.

>
> mem_cgroup_flush_stats() has some thresholding to make sure we don't
> do flushes unnecessarily, and I have a pending series in mm-unstable
> that makes that thresholding per-memcg. Keep in mind that adding a
> call to mem_cgroup_flush_stats() will cause a conflict in mm-unstable,
> because the series there adds a memcg argument to
> mem_cgroup_flush_stats(). That should be easily amenable though, I can
> post a fixlet for my series to add the memcg argument there on top of
> users if needed.

Hmm so how should we proceed from here? How about this:

a) I can send a fixlet to move the enablement check above the stats
flushing + use mem_cgroup_flush_stats
b) Then maybe, you can send a fixlet to update this new callsite?

Does that sound reasonable?

>
> >
> > Thanks!
> >
> > > +             return 0;
> > > +
> > > +     nr_protected =3D
> > > +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_p=
rotected);
> > > +     nr_freeable =3D list_lru_shrink_count(&pool->list_lru, sc);
> > > +     /*
> > > +      * Subtract the lru size by an estimate of the number of pages
> > > +      * that should be protected.
> > > +      */
> > > +     nr_freeable =3D nr_freeable > nr_protected ? nr_freeable - nr_p=
rotected : 0;
> > > +
> > > +     /*
> > > +      * Scale the number of freeable pages by the memory saving fact=
or.
> > > +      * This ensures that the better zswap compresses memory, the fe=
wer
> > > +      * pages we will evict to swap (as it will otherwise incur IO f=
or
> > > +      * relatively small memory saving).
> > > +      */
> > > +     return mult_frac(nr_freeable, nr_backing, nr_stored);
> > > +}
> > > +
> > > +static void zswap_alloc_shrinker(struct zswap_pool *pool)
> > > +{
> > > +     pool->shrinker =3D
> > > +             shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWA=
RE, "mm-zswap");
> > > +     if (!pool->shrinker)
> > > +             return;
> > > +
> > > +     pool->shrinker->private_data =3D pool;
> > > +     pool->shrinker->scan_objects =3D zswap_shrinker_scan;
> > > +     pool->shrinker->count_objects =3D zswap_shrinker_count;
> > > +     pool->shrinker->batch =3D 0;
> > > +     pool->shrinker->seeks =3D DEFAULT_SEEKS;
> > > +}
> > > +
> > >  /*********************************
> > >  * per-cpu code
> > >  **********************************/
> [..]