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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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