Re: [PATCH] memcg: remove unneeded preempt_disable

Greg Thelen
2011-08-19T00:00:58Z

On Thu, Aug 18, 2011 at 2:40 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> (cc linux-arch)
>
> On Wed, 17 Aug 2011 23:50:53 -0700
> Greg Thelen <gthelen@google.com> wrote:
>
>> Both mem_cgroup_charge_statistics() and mem_cgroup_move_account() were
>> unnecessarily disabling preemption when adjusting per-cpu counters:
>>     preempt_disable()
>>     __this_cpu_xxx()
>>     __this_cpu_yyy()
>>     preempt_enable()
>>
>> This change does not disable preemption and thus CPU switch is possible
>> within these routines.  This does not cause a problem because the total
>> of all cpu counters is summed when reporting stats.  Now both
>> mem_cgroup_charge_statistics() and mem_cgroup_move_account() look like:
>>     this_cpu_xxx()
>>     this_cpu_yyy()
>>
>> ...
>>
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -664,24 +664,20 @@ static unsigned long mem_cgroup_read_events(struct mem_cgroup *mem,
>>  static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
>>                                        bool file, int nr_pages)
>>  {
>> -     preempt_disable();
>> -
>>       if (file)
>> -             __this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_CACHE], nr_pages);
>> +             this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_CACHE], nr_pages);
>>       else
>> -             __this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_RSS], nr_pages);
>> +             this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_RSS], nr_pages);
>>
>>       /* pagein of a big page is an event. So, ignore page size */
>>       if (nr_pages > 0)
>> -             __this_cpu_inc(mem->stat->events[MEM_CGROUP_EVENTS_PGPGIN]);
>> +             this_cpu_inc(mem->stat->events[MEM_CGROUP_EVENTS_PGPGIN]);
>>       else {
>> -             __this_cpu_inc(mem->stat->events[MEM_CGROUP_EVENTS_PGPGOUT]);
>> +             this_cpu_inc(mem->stat->events[MEM_CGROUP_EVENTS_PGPGOUT]);
>>               nr_pages = -nr_pages; /* for event */
>>       }
>>
>> -     __this_cpu_add(mem->stat->events[MEM_CGROUP_EVENTS_COUNT], nr_pages);
>> -
>> -     preempt_enable();
>> +     this_cpu_add(mem->stat->events[MEM_CGROUP_EVENTS_COUNT], nr_pages);
>>  }
>
> On non-x86 architectures this_cpu_add() internally does
> preempt_disable() and preempt_enable().  So the patch is a small
> optimisation for x86 and a larger deoptimisation for non-x86.
>
> I think I'll apply it, as the call frequency is low (correct?) and the
> problem will correct itself as other architectures implement their
> atomic this_cpu_foo() operations.

mem_cgroup_charge_statistics() is a common operation, which is called
on each memcg page charge and uncharge.

The per arch/config effects of this patch:
* non-preemptible kernels: there's no difference before/after this patch.
* preemptible x86: this patch helps by removing an unnecessary
preempt_disable/enable.
* preemptible non-x86: this patch hurts by adding implicit
preempt_disable/enable around each operation.

So I am uncomfortable this patch's unmeasured impact on archs that do
not have atomic this_cpu_foo() operations. Please drop the patch from
mmotm. Sorry for the noise.