Re: [PATCH v9 03/13] memcg: add dirty page accounting infrastructure

[Greg Thelen <gthelen@google.com>]

KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:

> On Wed, 17 Aug 2011 09:14:55 -0700
> Greg Thelen <gthelen@google.com> wrote:
>
>> Add memcg routines to count dirty, writeback, and unstable_NFS pages.
>> These routines are not yet used by the kernel to count such pages.  A
>> later change adds kernel calls to these new routines.
>> 
>> As inode pages are marked dirty, if the dirtied page's cgroup differs
>> from the inode's cgroup, then mark the inode shared across several
>> cgroup.
>> 
>> Signed-off-by: Greg Thelen <gthelen@google.com>
>> Signed-off-by: Andrea Righi <andrea@betterlinux.com>
>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> A nitpick..
>
>
>
>> +static inline
>> +void mem_cgroup_move_account_page_stat(struct mem_cgroup *from,
>> +				       struct mem_cgroup *to,
>> +				       enum mem_cgroup_stat_index idx)
>> +{
>> +	preempt_disable();
>> +	__this_cpu_dec(from->stat->count[idx]);
>> +	__this_cpu_inc(to->stat->count[idx]);
>> +	preempt_enable();
>> +}
>> +
>
> this_cpu_dec()
> this_cpu_inc()
>
> without preempt_disable/enable will work. CPU change between dec/inc will
> not be problem.
>
> Thanks,
> -Kame

I agree, but this fix is general cleanup, which seems independent of
memcg dirty accounting.  This preemption disable/enable pattern exists
before this patch series in both mem_cgroup_charge_statistics() and
mem_cgroup_move_account().  For consistency we should change both.  To
keep the dirty page accounting series simple, I would like to make these
changes outside of this series.  On x86 usage of this_cpu_dec/inc looks
equivalent to __this_cpu_inc(), so I assume the only trade off is that
preemptible non-x86 using generic this_this_cpu() will internally
disable/enable preemption in this_cpu_*() operations.

I'll submit a cleanup patch outside of the dirty limit patches for this.

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