Re: [PATCH] mm/page_alloc: call check_new_pages() while zone spinlock is not held

[Vlastimil Babka]

On 3/4/22 03:09, Eric Dumazet wrote:
> On Thu, Mar 3, 2022 at 5:59 PM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>>
>> From: Eric Dumazet <edumazet@xxxxxxxxxx>
>>
>> For high order pages not using pcp, rmqueue() is currently calling
>> the costly check_new_pages() while zone spinlock is held.
>>
>> This is not needed, we can release the spinlock sooner to reduce
>> zone spinlock contention.
>>
>> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
>> Cc: Wei Xu <weixugc@xxxxxxxxxx>
>> Cc: Greg Thelen <gthelen@xxxxxxxxxx>
>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> Cc: David Rientjes <rientjes@xxxxxxxxxx>
>> ---
>>  mm/page_alloc.c | 17 +++++++----------
>>  1 file changed, 7 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 3589febc6d31928f850ebe5a4015ddc40e0469f3..0890a65f8cc2259e82bc1f5ba95a592fb30f9fb8 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3685,7 +3685,6 @@ struct page *rmqueue(struct zone *preferred_zone,
>>                         gfp_t gfp_flags, unsigned int alloc_flags,
>>                         int migratetype)
>>  {
>> -       unsigned long flags;
>>         struct page *page;
>>
>>         if (likely(pcp_allowed_order(order))) {
>> @@ -3706,10 +3705,12 @@ struct page *rmqueue(struct zone *preferred_zone,
>>          * allocate greater than order-1 page units with __GFP_NOFAIL.
>>          */
>>         WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
>> -       spin_lock_irqsave(&zone->lock, flags);
>>
>>         do {
>> +               unsigned long flags;
>> +
>>                 page = NULL;
>> +               spin_lock_irqsave(&zone->lock, flags);
>>                 /*
>>                  * order-0 request can reach here when the pcplist is skipped
>>                  * due to non-CMA allocation context. HIGHATOMIC area is
>> @@ -3723,13 +3724,13 @@ struct page *rmqueue(struct zone *preferred_zone,
>>                 }
>>                 if (!page)
>>                         page = __rmqueue(zone, order, migratetype, alloc_flags);
>> -       } while (page && check_new_pages(page, order));
>> -       if (!page)
>> -               goto failed;
>> +               spin_unlock_irqrestore(&zone->lock, flags);
>> +               if (!page)
>> +                       return NULL;
>> +       } while (check_new_pages(page, order));
>>
> 
> Oh well, it seems hard irqs have to be disabled when calling the
> following function.

Ah, you found out already. Well maybe it could simply be moved inside the
loop under the locked section and always done when page != NULL? I mean if
check_new_pages() fails we just leak the problematic pages anyway so they
are no longer free to allocate anymore and we should not count them as such.

> I will send a V2.
> 
>>         __mod_zone_freepage_state(zone, -(1 << order),
>>                                   get_pcppage_migratetype(page));
>> -       spin_unlock_irqrestore(&zone->lock, flags);
>>
>>         __count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
>>         zone_statistics(preferred_zone, zone, 1);
>> @@ -3743,10 +3744,6 @@ struct page *rmqueue(struct zone *preferred_zone,
>>
>>         VM_BUG_ON_PAGE(page && bad_range(zone, page), page);
>>         return page;
>> -
>> -failed:
>> -       spin_unlock_irqrestore(&zone->lock, flags);
>> -       return NULL;
>>  }
>>
>>  #ifdef CONFIG_FAIL_PAGE_ALLOC
>> --
>> 2.35.1.616.g0bdcbb4464-goog
>>