Message-ID: <CA+CLi1i02o+ZzWEgK-U=wftqQ9DgijZWUnEAeGNAZ17SeBh9ig@mail.gmail.com>
Date:   Fri, 26 May 2023 10:52:20 +0200
From:   Domenico Cerasuolo <cerasuolodomenico@...il.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com,
        hannes@...xchg.org, kernel-team@...com
Subject: Re: [PATCH] mm: zswap: shrink until can accept

On Thu, May 25, 2023 at 9:10 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Thu, May 25, 2023 at 9:53 AM Domenico Cerasuolo
> <cerasuolodomenico@...il.com> wrote:
> >
> > On Thu, May 25, 2023 at 2:59 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > >
> > > Hi Domenico,
> > >
> > > On Tue, May 23, 2023 at 11:50 PM Domenico Cerasuolo
> > > <cerasuolodomenico@...il.com> wrote:
> > > >
> > > > This update addresses an issue with the zswap reclaim mechanism, which
> > > > hinders the efficient offloading of cold pages to disk, thereby
> > > > compromising the preservation of the LRU order and consequently
> > > > diminishing, if not inverting, its performance benefits.
> > > >
> > > > The functioning of the zswap shrink worker was found to be inadequate,
> > > > as shown by basic benchmark test. For the test, a kernel build was
> > > > utilized as a reference, with its memory confined to 1G via a cgroup and
> > > > a 5G swap file provided. The results are presented below, these are
> > > > averages of three runs without the use of zswap:
> > > >
> > > > real 46m26s
> > > > user 35m4s
> > > > sys 7m37s
> > > >
> > > > With zswap (zbud) enabled and max_pool_percent set to 1 (in a 32G
> > > > system), the results changed to:
> > > >
> > > > real 56m4s
> > > > user 35m13s
> > > > sys 8m43s
> > > >
> > > > written_back_pages: 18
> > > > reject_reclaim_fail: 0
> > > > pool_limit_hit:1478
> > > >
> > > > Besides the evident regression, one thing to notice from this data is
> > > > the extremely low number of written_back_pages and pool_limit_hit.
> > > >
> > > > The pool_limit_hit counter, which is increased in zswap_frontswap_store
> > > > when zswap is completely full, doesn't account for a particular
> > > > scenario: once zswap hits his limit, zswap_pool_reached_full is set to
> > > > true; with this flag on, zswap_frontswap_store rejects pages if zswap is
> > > > still above the acceptance threshold. Once we include the rejections due
> > > > to zswap_pool_reached_full && !zswap_can_accept(), the number goes from
> > > > 1478 to a significant 21578266.
> > > >
> > > > Zswap is stuck in an undesirable state where it rejects pages because
> > > > it's above the acceptance threshold, yet fails to attempt memory
> > > > reclaimation. This happens because the shrink work is only queued when
> > > > zswap_frontswap_store detects that it's full and the work itself only
> > > > reclaims one page per run.
> > > >
> > > > This state results in hot pages getting written directly to disk,
> > > > while cold ones remain memory, waiting only to be invalidated. The LRU
> > > > order is completely broken and zswap ends up being just an overhead
> > > > without providing any benefits.
> > > >
> > > > This commit applies 2 changes: a) the shrink worker is set to reclaim
> > > > pages until the acceptance threshold is met and b) the task is also
> > > > enqueued when zswap is not full but still above the threshold.
> > > >
> > > > Testing this suggested update showed much better numbers:
> > > >
> > > > real 36m37s
> > > > user 35m8s
> > > > sys 9m32s
> > > >
> > > > written_back_pages: 10459423
> > > > reject_reclaim_fail: 12896
> > > > pool_limit_hit: 75653
> > >
> > > Impressive numbers, and great find in general!
> > >
> > > I wonder how other workloads benefit/regress from this change.
> > > Anything else that you happened to run? :)
> > >
> > Hi Yosry,
> >
> > thanks for the quick feedback!
> >
> > Besides stressers, I haven't tested any other actual workload, we don't have
> > writeback in production yet so I can't provide any data from there. I was
> > wondering what kind of actual workload I could use to test this on my desktop,
> > but I couldn't think of anything relevant, I'm open to suggestions though :)
>
> Nothing in mind in particular as well. Perhaps others have ideas.
>
> > > >
> > > > Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit")
> > > > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@...il.com>
> > > > ---
> > > >  mm/zswap.c | 10 +++++++---
> > > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > > index 59da2a415fbb..2ee0775d8213 100644
> > > > --- a/mm/zswap.c
> > > > +++ b/mm/zswap.c
> > > > @@ -587,9 +587,13 @@ static void shrink_worker(struct work_struct *w)
> > > >  {
> > > >         struct zswap_pool *pool = container_of(w, typeof(*pool),
> > > >                                                 shrink_work);
> > > > +       int ret;
> > > >
> > > > -       if (zpool_shrink(pool->zpool, 1, NULL))
> > > > -               zswap_reject_reclaim_fail++;
> > > > +       do {
> > > > +               ret = zpool_shrink(pool->zpool, 1, NULL);
> > > > +               if (ret)
> > > > +                       zswap_reject_reclaim_fail++;
> > > > +       } while (!zswap_can_accept() && ret != -EINVAL);
> > >
> > > One question/comment here about the retry logic.
> > >
> > > So I looked through the awfully convoluted writeback code, and there
> > > are multiple layers, and some of them tend to overwrite the return
> > > value of the layer underneath :(
> > >
> > > For zsmalloc (as an example):
> > > zpool_shrink()->zs_zpool_shrink()->zs_reclaim_page()->zpool_ops.evict()->zswap_writeback_entry().
> > >
> > > First of all, that zpool_ops is an unnecessarily confusing
> > > indirection, but anyway.
> > >
> > > - zswap_writeback_entry() will either return -ENOMEM or -EEXIST on error
> > > - zs_reclaim_page()/zbud_reclaim_page() will return -EINVAL if the lru
> > > is empty, and -EAGAIN on other errors.
> > > - zs_zpool_shrink()/zbud_zpool_shrink() will mostly propagate the
> > > return value of  zs_reclaim_page()/zbud_reclaim_page().
> > > - zpool_shrink() will return -EINVAL if the driver does not support
> > > shrinking, otherwise it will propagate the return value from the
> > > driver.
> > >
> > > So it looks like we will get -EINVAL only if the driver lru is empty
> > > or the driver does not support writeback, so rightfully we should not
> > > retry.
> > >
> > > If zswap_writeback_entry() returns -EEXIST, it probably means that we
> > > raced with another task decompressing the page, so rightfully we
> > > should retry to writeback another page instead.
> > >
> > > If zswap_writeback_entry() returns -ENOMEM, it doesn't necessarily
> > > mean that we couldn't allocate memory (unfortunately), it looks like
> > > we will return -ENOMEM in other cases as well. Arguably, we can retry
> > > in all cases, because even if we were actually out of memory, we are
> > > trying to make an allocation that will eventually free more memory.
> > >
> > > In all cases, I think it would be nicer if we retry if ret == -EAGAIN
> > > instead. It is semantically more sane. In this specific case it is
> > > functionally NOP as zs_reclaim_page()/zbud_reclaim_page() will mostly
> > > return -EAGAIN anyway, but in case someone tries to use saner errnos
> > > in the future, this will age better.
> > Retrying if ret == -EAGAIN seems much nicer indeed, will change it.
> > >
> > > Also, do we intentionally want to keep retrying forever on failure? Do
> > > we want to add a max number of retries?
> > If the drivers guaranteed that zpool_shrink will remove at least an entry
> > from their LRU, the retry wouldn't be needed because the LRU will
> > eventually be emptied. But this is an assumption on the implementation of
>
> I don't think any zpool driver can guarantee to writeback at least one
> page. It can fail for reasons beyond their control (e.g. cannot
> allocate a page to decompress to).
>
> > the zpool, so yes, we could use a max retries. I think that being a sanity
> > check, it should overshoot the required number of iterations in order to
> > avoid a premature break, what about retrying a max of
> > zswap_stored_pages times?
>
> Why is it just a sanity check? Consider a case where the system is
> under extreme memory pressure that the drivers cannot allocate pages
> to decompress to. The drivers would be continuously competing with all
> other allocations on the machine. I think we should give up at some
> point. In any case, you changed the zswap_frontswap_store() to goto
> shrink if !zswap_can_accept(), so next time we try to compress a page
> to zswap we will invoke try again anyway -- perhaps under better
> circumstances.
>
> I think zswap_stored_pages is too much, keep in mind that it also
> includes same-filled pages which are not even stored in the zpool
> drivers. Maybe we should allow a fixed number of failures. If
> zpool_shrink() is successful, keep going until zswap_can_accept().
> Otherwise allow a fixed number of failures before giving up.
>

Yes, I think I got carried away by a writeback frenzy, while testing the LRU
refactor I had to fight to get a decent amount of writebacks, and ended up
being too afraid the shrink work would stop.
What about using MAX_RECLAIM_RETRIES? I like the idea of putting a limit but
I wouldn't know how to pick a fixed number.

> Maybe we can improve the error codes propagated through the writeback
> code as well to improve the retry logic. For example, if
> zswap_writeback_entry() returns EEXIST then retrying should be
> harmless, but ENOMEM might be harmful. Both of which are propagated as
> EAGAIN from zs_zpool_shrink()/zbud_zpool_shrink().
>

That would be a nice improvement indeed, I'd queue it after the LRU refactor
though.

> > >
> > > >         zswap_pool_put(pool);
> > > >  }
> > > >
> > > > @@ -1188,7 +1192,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> > > >         if (zswap_pool_reached_full) {
> > > >                if (!zswap_can_accept()) {
> > > >                         ret = -ENOMEM;
> > > > -                       goto reject;
> > > > +                       goto shrink;
> > > >                 } else
> > > >                         zswap_pool_reached_full = false;
> > > >         }
> > > > --
> > > > 2.34.1
> > > >

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