Re: [PATCH] mm: zswap: shrink until can accept

[Yosry Ahmed]

On Thu, May 25, 2023 at 9:53 AM Domenico Cerasuolo
<cerasuolodomenico@xxxxxxxxx> wrote:
>
> On Thu, May 25, 2023 at 2:59 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> > Hi Domenico,
> >
> > On Tue, May 23, 2023 at 11:50 PM Domenico Cerasuolo
> > <cerasuolodomenico@xxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > >  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.

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().

> >
> > >         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
> > >