On Wed, Oct 2, 2024 at 8:43 AM Huang, Ying ying.huang@intel.com wrote:
Barry Song 21cnbao@gmail.com writes:
On Tue, Oct 1, 2024 at 7:43 AM Huang, Ying ying.huang@intel.com wrote:
Barry Song 21cnbao@gmail.com writes:
On Sun, Sep 29, 2024 at 3:43 PM Huang, Ying ying.huang@intel.com wrote:
Hi, Barry,
Barry Song 21cnbao@gmail.com writes:
From: Barry Song v-songbaohua@oppo.com
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@worktop.programming.kick...
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;