Re: [RFC][PATCH v7 08/14] writeback: add memcg fields to writeback_control

[Greg Thelen <gthelen@google.com>]

On Fri, May 13, 2011 at 2:41 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Fri, 13 May 2011 01:47:47 -0700
> Greg Thelen <gthelen@google.com> wrote:
>
>> Add writeback_control fields to differentiate between bdi-wide and
>> per-cgroup writeback.  Cgroup writeback is also able to differentiate
>> between writing inodes isolated to a particular cgroup and inodes shared
>> by multiple cgroups.
>>
>> Signed-off-by: Greg Thelen <gthelen@google.com>
>
> Personally, I want to see new flags with their usage in a patch...

Ok.  Next version will merge the flag definition with first usage of the flag.

>> ---
>>  include/linux/writeback.h |    2 ++
>>  1 files changed, 2 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
>> index d10d133..4f5c0d2 100644
>> --- a/include/linux/writeback.h
>> +++ b/include/linux/writeback.h
>> @@ -47,6 +47,8 @@ struct writeback_control {
>>       unsigned for_reclaim:1;         /* Invoked from the page allocator */
>>       unsigned range_cyclic:1;        /* range_start is cyclic */
>>       unsigned more_io:1;             /* more io to be dispatched */
>> +     unsigned for_cgroup:1;          /* enable cgroup writeback */
>> +     unsigned shared_inodes:1;       /* write inodes spanning cgroups */
>>  };
>
>
> If shared_inode is really rare case...we don't need to have this shared_inodes
> flag and do writeback shared_inode always.....No ?
>
> Thanks,
> -Kame

The shared_inodes field is present to avoid punishing cgroups that are
not sharing, if they are run on a system that also includes sharing.

This issue is being debated in another thread: "[RFC][PATCH v7 00/14]
memcg: per cgroup dirty page accounting".  Depending on the decision,
we may be able to delete the shared_inode fields if we choose to
always write shared inodes in both cgroup foreground and cgroup
background writeback.

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