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Subject: Re: [PATCH RESEND] mm/page_alloc: skip setting nodemask when we are
 in interrupt
To:     Pekka Enberg <penberg@gmail.com>,
        Muchun Song <songmuchun@bytedance.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20200703061350.94474-1-songmuchun@bytedance.com>
 <CAOJsxLGc-o1Os7HAoQmhd1DNxGWVrfbw2R8NroTQg0wHNyWX1w@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <fe891223-9f83-d112-b185-643d6121712d@redhat.com>
Date:   Fri, 3 Jul 2020 09:20:27 +0200
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On 03.07.20 08:34, Pekka Enberg wrote:
> On Fri, Jul 3, 2020 at 9:14 AM Muchun Song <songmuchun@bytedance.com> wrote:
>>
>> When we are in the interrupt context, it is irrelevant to the
>> current task context. If we use current task's mems_allowed, we
>> can fair to alloc pages in the fast path and fall back to slow
>> path memory allocation when the current node(which is the current
>> task mems_allowed) does not have enough memory to allocate. In
>> this case, it slows down the memory allocation speed of interrupt
>> context. So we can skip setting the nodemask to allow any node
>> to allocate memory, so that fast path allocation can success.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>  mm/page_alloc.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index b48336e20bdcd..a6c36cd557d1d 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4726,10 +4726,12 @@ static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order,
>>
>>         if (cpusets_enabled()) {
>>                 *alloc_mask |= __GFP_HARDWALL;
>> -               if (!ac->nodemask)
>> -                       ac->nodemask = &cpuset_current_mems_allowed;
>> -               else
>> +               if (!ac->nodemask) {
>> +                       if (!in_interrupt())
>> +                               ac->nodemask = &cpuset_current_mems_allowed;
> 
> If !ac->nodemask and in_interrupt() the ALLOC_CPUSET flag is not set,
> which by-passes the __cpuset_zone_allowed() check for allocations.
> This works fine because in the case if in_interrupt() the function
> allows allocation on any zone/node.
> 
>> +               } else {
>>                         *alloc_flags |= ALLOC_CPUSET;
>> +               }
>>         }
> 
> However, if you write the condition as follows:
> 
>         if (cpusets_enabled()) {
>                 *alloc_mask |= __GFP_HARDWALL;
>                 if (!in_interrupt() && !ac->nodemask)
>                         ac->nodemask = &cpuset_current_mems_allowed;
>                 else
>                         *alloc_flags |= ALLOC_CPUSET;
>         }

^ looks much cleaner as well. Do we want to add a summarizing comment?

> 
> then the code is future-proof in case of __cpuset_zone_allowed() is
> one day extended to support IRQ context too (it probably should
> eventually respect IRQ SMP affinity).


-- 
Thanks,

David / dhildenb