On Tue, Sep 6, 2011 at 2:58 AM, Johannes Weiner <jweiner@xxxxxxxxxx> wrote:
> On Wed, Aug 17, 2011 at 11:50:53PM -0700, Greg Thelen 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()
>>
>> Reported-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
>
> I just noticed that both cases have preemption disabled anyway because
> of the page_cgroup bit spinlock.
>
> So removing the preempt_disable() is fine but we can even keep the
> non-atomic __this_cpu operations.
>
> Something like this instead?
>
> ---
> From: Johannes Weiner <jweiner@xxxxxxxxxx>
> Subject: mm: memcg: remove needless recursive preemption disabling
>
> Callsites of mem_cgroup_charge_statistics() hold the page_cgroup bit
> spinlock, which implies disabled preemption.
>
> The same goes for the explicit preemption disabling to account mapped
> file pages in mem_cgroup_move_account().
>
> The explicit disabling of preemption in both cases is redundant.
>
> Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
Looks good, thanks.
Reviewed-by: Greg Thelen <gthelen@xxxxxxxxxx>
> ---
> mm/memcontrol.c | 6 ------
> 1 file changed, 6 deletions(-)
>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -618,8 +618,6 @@ static unsigned long mem_cgroup_read_eve
> 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);
> else
> @@ -634,8 +632,6 @@ static void mem_cgroup_charge_statistics
> }
>
> __this_cpu_add(mem->stat->events[MEM_CGROUP_EVENTS_COUNT], nr_pages);
> -
> - preempt_enable();
> }
>
> unsigned long
> @@ -2582,10 +2578,8 @@ static int mem_cgroup_move_account(struc
>
> if (PageCgroupFileMapped(pc)) {
> /* Update mapped_file data for mem_cgroup */
> - preempt_disable();
> __this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> __this_cpu_inc(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> - preempt_enable();
> }
> mem_cgroup_charge_statistics(from, PageCgroupCache(pc), -nr_pages);
> if (uncharge)
>
--
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