Re: [PATCH v8 3/6] zswap: make shrinking memcg-aware

Yosry Ahmed
2023-12-05T19:00:24Z

[..]
> > > static void shrink_worker(struct work_struct *w)
> > > {
> > > struct zswap_pool *pool = container_of(w, typeof(*pool),
> > > shrink_work);
> > > + struct mem_cgroup *memcg;
> > > int ret, failures = 0;
> > >
> > > + /* global reclaim will select cgroup in a round-robin fashion. */
> > > do {
> > > - ret = zswap_reclaim_entry(pool);
> > > - if (ret) {
> > > - zswap_reject_reclaim_fail++;
> > > - if (ret != -EAGAIN)
> > > + spin_lock(&zswap_pools_lock);
> > > + pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
> > > + memcg = pool->next_shrink;
> > > +
> > > + /*
> > > + * We need to retry if we have gone through a full round trip, or if we
> > > + * got an offline memcg (or else we risk undoing the effect of the
> > > + * zswap memcg offlining cleanup callback). This is not catastrophic
> > > + * per se, but it will keep the now offlined memcg hostage for a while.
> > > + *
> > > + * Note that if we got an online memcg, we will keep the extra
> > > + * reference in case the original reference obtained by mem_cgroup_iter
> > > + * is dropped by the zswap memcg offlining callback, ensuring that the
> > > + * memcg is not killed when we are reclaiming.
> > > + */
> > > + if (!memcg) {
> > > + spin_unlock(&zswap_pools_lock);
> > > + if (++failures == MAX_RECLAIM_RETRIES)
> > > break;
> > > +
> > > + goto resched;
> > > + }
> > > +
> > > + if (!mem_cgroup_online(memcg)) {
> > > + /* drop the reference from mem_cgroup_iter() */
> > > + mem_cgroup_put(memcg);
> >
> > Probably better to use mem_cgroup_iter_break() here?
>
> mem_cgroup_iter_break(NULL, memcg) seems to perform the same thing, right?

Yes, but it's better to break the iteration with the documented API
(e.g. if mem_cgroup_iter_break() changes to do extra work).

>
> >
> > Also, I don't see mem_cgroup_tryget_online() being used here (where I
> > expected it to be used), did I miss it?
>
> Oh shoot yeah that was a typo - it should be
> mem_cgroup_tryget_online(). Let me send a fix to that.
>
> >
> > > + pool->next_shrink = NULL;
> > > + spin_unlock(&zswap_pools_lock);
> > > +
> > > if (++failures == MAX_RECLAIM_RETRIES)
> > > break;
> > > +
> > > + goto resched;
> > > }
> > > + spin_unlock(&zswap_pools_lock);
> > > +
> > > + ret = shrink_memcg(memcg);
> >
> > We just checked for online-ness above, and then shrink_memcg() checks
> > it again. Is this intentional?
>
> Hmm these two checks are for two different purposes. The check above
> is mainly to prevent accidentally undoing the offline cleanup callback
> during memcg selection step. Inside shrink_memcg(), we check
> onlineness again to prevent reclaiming from offlined memcgs - which in
> effect will trigger the reclaim of the parent's memcg.

Right, but two checks in close proximity are not doing a lot.
Especially that the memcg online-ness can change right after the check
inside shrink_memcg() anyway, so it's a best effort thing.

Anyway, it shouldn't matter much. We can leave it.

>
> >
> > > + /* drop the extra reference */
> >
> > Where does the extra reference come from?
>
> The extra reference is from mem_cgroup_tryget_online(). We get two
> references in the dance above - one from mem_cgroup_iter() (which can
> be dropped) and one extra from mem_cgroup_tryget_online(). I kept the
> second one in case the first one was dropped by the zswap memcg
> offlining callback, but after reclaiming it is safe to just drop it.

Right. I was confused by the missing mem_cgroup_tryget_online().

>
> >
> > > + mem_cgroup_put(memcg);
> > > +
> > > + if (ret == -EINVAL)
> > > + break;
> > > + if (ret && ++failures == MAX_RECLAIM_RETRIES)
> > > + break;
> > > +
> > > +resched:
> > > cond_resched();
> > > } while (!zswap_can_accept());
> > > - zswap_pool_put(pool);
> > > }
> > >
> > > static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
[..]
> > > @@ -1240,15 +1395,15 @@ bool zswap_store(struct folio *folio)
> > > zswap_invalidate_entry(tree, dupentry);
> > > }
> > > spin_unlock(&tree->lock);
> > > -
> > > - /*
> > > - * XXX: zswap reclaim does not work with cgroups yet. Without a
> > > - * cgroup-aware entry LRU, we will push out entries system-wide based on
> > > - * local cgroup limits.
> > > - */
> > > objcg = get_obj_cgroup_from_folio(folio);
> > > - if (objcg && !obj_cgroup_may_zswap(objcg))
> > > - goto reject;
> > > + if (objcg && !obj_cgroup_may_zswap(objcg)) {
> > > + memcg = get_mem_cgroup_from_objcg(objcg);
> >
> > Do we need a reference here? IIUC, this is folio_memcg() and the folio
> > is locked, so folio_memcg() should remain stable, no?
>
> Hmmm obj_cgroup_may_zswap() also holds a reference to the objcg's
> memcg, so I just followed the patterns to be safe.

Perhaps it's less clear inside obj_cgroup_may_zswap(). We can actually
pass the folio to obj_cgroup_may_zswap(), add a debug check that the
folio is locked, and avoid getting the ref there as well. That can be
done separately. Perhaps Johannes can shed some light on this, if
there's a different reason why getting a ref there is needed.

For this change, I think the refcount manipulation is unnecessary.

>
>
> >
> > Same for the call below.
> >
> > > + if (shrink_memcg(memcg)) {
> > > + mem_cgroup_put(memcg);
> > > + goto reject;
> > > + }
> > > + mem_cgroup_put(memcg);
> > > + }
> > >
> > > /* reclaim space if needed */
> > > if (zswap_is_full()) {
[..]