On 03.07.20 08:34, Pekka Enberg wrote:
> On Fri, Jul 3, 2020 at 9:14 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>> ---
>> 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
