Re: [PATCH] mm: avoid unconditional one-tick sleep when swapcache_prepare fails

[Chris Li]

On Tue, Oct 1, 2024 at 6:58 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Wed, Oct 2, 2024 at 8:43 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >
> > Barry Song <21cnbao@xxxxxxxxx> writes:
> >
> > > On Tue, Oct 1, 2024 at 7:43 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> > >>
> > >> Barry Song <21cnbao@xxxxxxxxx> writes:
> > >>
> > >> > On Sun, Sep 29, 2024 at 3:43 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> > >> >>
> > >> >> Hi, Barry,
> > >> >>
> > >> >> Barry Song <21cnbao@xxxxxxxxx> writes:
> > >> >>
> > >> >> > From: Barry Song <v-songbaohua@xxxxxxxx>
> > >> >> >
> > >> >> > Commit 13ddaf26be32 ("mm/swap: fix race when skipping swapcache")
> > >> >> > introduced an unconditional one-tick sleep when `swapcache_prepare()`
> > >> >> > fails, which has led to reports of UI stuttering on latency-sensitive
> > >> >> > Android devices. To address this, we can use a waitqueue to wake up
> > >> >> > tasks that fail `swapcache_prepare()` sooner, instead of always
> > >> >> > sleeping for a full tick. While tasks may occasionally be woken by an
> > >> >> > unrelated `do_swap_page()`, this method is preferable to two scenarios:
> > >> >> > rapid re-entry into page faults, which can cause livelocks, and
> > >> >> > multiple millisecond sleeps, which visibly degrade user experience.
> > >> >>
> > >> >> In general, I think that this works.  Why not extend the solution to
> > >> >> cover schedule_timeout_uninterruptible() in __read_swap_cache_async()
> > >> >> too?  We can call wake_up() when we clear SWAP_HAS_CACHE.  To avoid
> > >> >
> > >> > Hi Ying,
> > >> > Thanks for your comments.
> > >> > I feel extending the solution to __read_swap_cache_async() should be done
> > >> > in a separate patch. On phones, I've never encountered any issues reported
> > >> > on that path, so it might be better suited for an optimization rather than a
> > >> > hotfix?
> > >>
> > >> Yes.  It's fine to do that in another patch as optimization.
> > >
> > > Ok. I'll prepare a separate patch for optimizing that path.
> >
> > Thanks!
> >
> > >>
> > >> >> overhead to call wake_up() when there's no task waiting, we can use an
> > >> >> atomic to count waiting tasks.
> > >> >
> > >> > I'm not sure it's worth adding the complexity, as wake_up() on an empty
> > >> > waitqueue should have a very low cost on its own?
> > >>
> > >> wake_up() needs to call spin_lock_irqsave() unconditionally on a global
> > >> shared lock.  On systems with many CPUs (such servers), this may cause
> > >> severe lock contention.  Even the cache ping-pong may hurt performance
> > >> much.
> > >
> > > I understand that cache synchronization was a significant issue before
> > > qspinlock, but it seems to be less of a concern after its implementation.
> >
> > Unfortunately, qspinlock cannot eliminate cache ping-pong issue, as
> > discussed in the following thread.
> >
> > https://lore.kernel.org/lkml/20220510192708.GQ76023@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> >
> > > However, using a global atomic variable would still trigger cache broadcasts,
> > > correct?
> >
> > We can only change the atomic variable to non-zero when
> > swapcache_prepare() returns non-zero, and call wake_up() when the atomic
> > variable is non-zero.  Because swapcache_prepare() returns 0 most times,
> > the atomic variable is 0 most times.  If we don't change the value of
> > atomic variable, cache ping-pong will not be triggered.
>
> yes. this can be implemented by adding another atomic variable.
>
> >
> > Hi, Kairui,
> >
> > Do you have some test cases to test parallel zram swap-in?  If so, that
> > can be used to verify whether cache ping-pong is an issue and whether it
> > can be fixed via a global atomic variable.
> >
>
> Yes, Kairui please run a test on your machine with lots of cores before
> and after adding a global atomic variable as suggested by Ying. I am
> sorry I don't have a server machine.
>
> if it turns out you find cache ping-pong can be an issue, another
> approach would be a waitqueue hash:
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 2366578015ad..aae0e532d8b6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4192,6 +4192,23 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
> +/*
> + * Alleviating the 'thundering herd' phenomenon using a waitqueue hash
> + * when multiple do_swap_page() operations occur simultaneously.
> + */
> +#define SWAPCACHE_WAIT_TABLE_BITS 5
> +#define SWAPCACHE_WAIT_TABLE_SIZE (1 << SWAPCACHE_WAIT_TABLE_BITS)
> +static wait_queue_head_t swapcache_wqs[SWAPCACHE_WAIT_TABLE_SIZE];
> +
> +static int __init swapcache_wqs_init(void)
> +{
> +       for (int i = 0; i < SWAPCACHE_WAIT_TABLE_SIZE; i++)
> +               init_waitqueue_head(&swapcache_wqs[i]);
> +
> +        return 0;
> +}
> +late_initcall(swapcache_wqs_init);
> +
>  /*
>   * We enter with non-exclusive mmap_lock (to exclude vma changes,
>   * but allow concurrent faults), and pte mapped but not yet locked.
> @@ -4204,6 +4221,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  {
>         struct vm_area_struct *vma = vmf->vma;
>         struct folio *swapcache, *folio = NULL;
> +       DECLARE_WAITQUEUE(wait, current);
> +       wait_queue_head_t *swapcache_wq;
>         struct page *page;
>         struct swap_info_struct *si = NULL;
>         rmap_t rmap_flags = RMAP_NONE;
> @@ -4297,12 +4316,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>                                  * undetectable as pte_same() returns true due
>                                  * to entry reuse.
>                                  */
> +                               swapcache_wq = &swapcache_wqs[hash_long(vmf->address & PMD_MASK,
> +                                                       SWAPCACHE_WAIT_TABLE_BITS)];

It is better to hash against the swap entry value rather than the
fault address. Same swap entries can map to different parts of the
page table. I am not sure this is triggerable in the SYNC_IO page
fault path, hash against the swap entries is more obviously correct.

Chris

>                                 if (swapcache_prepare(entry, nr_pages)) {
>                                         /*
>                                          * Relax a bit to prevent rapid
>                                          * repeated page faults.
>                                          */
> +                                       add_wait_queue(swapcache_wq, &wait);
>                                         schedule_timeout_uninterruptible(1);
> +                                       remove_wait_queue(swapcache_wq, &wait);
>                                         goto out_page;
>                                 }
>                                 need_clear_cache = true;
> @@ -4609,8 +4632,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>                 pte_unmap_unlock(vmf->pte, vmf->ptl);
>  out:
>         /* Clear the swap cache pin for direct swapin after PTL unlock */
> -       if (need_clear_cache)
> +       if (need_clear_cache) {
>                 swapcache_clear(si, entry, nr_pages);
> +               wake_up(swapcache_wq);
> +       }
>         if (si)
>                 put_swap_device(si);
>         return ret;
> @@ -4625,8 +4650,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>                 folio_unlock(swapcache);
>                 folio_put(swapcache);
>         }
> -       if (need_clear_cache)
> +       if (need_clear_cache) {
>                 swapcache_clear(si, entry, nr_pages);
> +               wake_up(swapcache_wq);
> +       }
>         if (si)
>                 put_swap_device(si);
>         return ret;
> --
> 2.34.1
>
> > --
> > Best Regards,
> > Huang, Ying
>
> Thanks
> Barry