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

[Barry Song]

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)];
 				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