Re: [PATCH v2 1/2] THP: avoid lock when check whether THP is in deferred list

Kirill A. Shutemov
2023-04-28T14:02:49Z

On Fri, Apr 28, 2023 at 02:28:07PM +0800, Yin, Fengwei wrote:
> Hi Kirill,
>
> On 4/25/2023 8:38 PM, Kirill A. Shutemov wrote:
> > On Tue, Apr 25, 2023 at 04:46:26PM +0800, Yin Fengwei wrote:
> >> free_transhuge_page() acquires split queue lock then check
> >> whether the THP was added to deferred list or not.
> >>
> >> It's safe to check whether the THP is in deferred list or not.
> >> When code hit free_transhuge_page(), there is no one tries
> >> to update the folio's _deferred_list.
> >>
> >> If folio is not in deferred_list, it's safe to check without
> >> acquiring lock.
> >>
> >> If folio is in deferred_list, the other node in deferred_list
> >> adding/deleteing doesn't impact the return value of
> >> list_epmty(@folio->_deferred_list).
> >
> > Typo.
> >
> >>
> >> Running page_fault1 of will-it-scale + order 2 folio for anonymous
> >> mapping with 96 processes on an Ice Lake 48C/96T test box, we could
> >> see the 61% split_queue_lock contention:
> >> - 71.28% 0.35% page_fault1_pro [kernel.kallsyms] [k]
> >> release_pages
> >> - 70.93% release_pages
> >> - 61.42% free_transhuge_page
> >> + 60.77% _raw_spin_lock_irqsave
> >>
> >> With this patch applied, the split_queue_lock contention is less
> >> than 1%.
> >>
> >> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
> >> Tested-by: Ryan Roberts <ryan.roberts@arm.com>
> >> ---
> >> mm/huge_memory.c | 19 ++++++++++++++++---
> >> 1 file changed, 16 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 032fb0ef9cd1..c620f1f12247 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -2799,12 +2799,25 @@ void free_transhuge_page(struct page *page)
> >> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
> >> unsigned long flags;
> >>
> >> - spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >> - if (!list_empty(&folio->_deferred_list)) {
> >> + /*
> >> + * At this point, there is no one trying to queue the folio
> >> + * to deferred_list. folio->_deferred_list is not possible
> >> + * being updated.
> >> + *
> >> + * If folio is already added to deferred_list, add/delete to/from
> >> + * deferred_list will not impact list_empty(&folio->_deferred_list).
> >> + * It's safe to check list_empty(&folio->_deferred_list) without
> >> + * acquiring the lock.
> >> + *
> >> + * If folio is not in deferred_list, it's safe to check without
> >> + * acquiring the lock.
> >> + */
> >> + if (data_race(!list_empty(&folio->_deferred_list))) {
> >> + spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >
> > Recheck under lock?
> In function deferred_split_scan(), there is following code block:
> if (folio_try_get(folio)) {
> list_move(&folio->_deferred_list, &list);
> } else {
> /* We lost race with folio_put() */
> list_del_init(&folio->_deferred_list);
> ds_queue->split_queue_len--;
> }
>
> I am wondering what kind of "lost race with folio_put()" can be.
>
> My understanding is that it's not necessary to handle this case here
> because free_transhuge_page() will handle it once folio get zero ref.
> But I must miss something here. Thanks.

free_transhuge_page() got when refcount is already zero. Both
deferred_split_scan() and free_transhuge_page() can see the page with zero
refcount. The check makes deferred_split_scan() to leave the page to the
free_transhuge_page().

--
Kiryl Shutsemau / Kirill A. Shutemov