On Thu, Aug 18, 2011 at 2:40 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> (cc linux-arch)
>
> On Wed, 17 Aug 2011 23:50:53 -0700
> Greg Thelen <gthelen@xxxxxxxxxx> 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.
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