Re: [PATCH v8 11/12] writeback: make background writeback cgroup aware

[Greg Thelen]

On Tue, Jun 7, 2011 at 5:18 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Tue, 7 Jun 2011 17:05:40 -0400
> Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
>> On Tue, Jun 07, 2011 at 01:43:08PM -0700, Greg Thelen wrote:
>> > Vivek Goyal <vgoyal@xxxxxxxxxx> writes:
>> >
>> > > On Fri, Jun 03, 2011 at 09:12:17AM -0700, Greg Thelen wrote:
>> > >> When the system is under background dirty memory threshold but a cgroup
>> > >> is over its background dirty memory threshold, then only writeback
>> > >> inodes associated with the over-limit cgroup(s).
>> > >>
>> > >
>> > > [..]
>> > >> -static inline bool over_bground_thresh(void)
>> > >> +static inline bool over_bground_thresh(struct bdi_writeback *wb,
>> > >> +                                       struct writeback_control *wbc)
>> > >>  {
>> > >>          unsigned long background_thresh, dirty_thresh;
>> > >>
>> > >>          global_dirty_limits(&background_thresh, &dirty_thresh);
>> > >>
>> > >> -        return (global_page_state(NR_FILE_DIRTY) +
>> > >> -                global_page_state(NR_UNSTABLE_NFS) > background_thresh);
>> > >> +        if (global_page_state(NR_FILE_DIRTY) +
>> > >> +            global_page_state(NR_UNSTABLE_NFS) > background_thresh) {
>> > >> +                wbc->for_cgroup = 0;
>> > >> +                return true;
>> > >> +        }
>> > >> +
>> > >> +        wbc->for_cgroup = 1;
>> > >> +        wbc->shared_inodes = 1;
>> > >> +        return mem_cgroups_over_bground_dirty_thresh();
>> > >>  }
>> > >
>> > > Hi Greg,
>> > >
>> > > So all the logic of writeout from mem cgroup works only if system is
>> > > below background limit. The moment we cross background limit, looks
>> > > like we will fall back to existing way of writting inodes?
>> >
>> > Correct.  If the system is over its background limit then the previous
>> > cgroup-unaware background writeback occurs.  I think of the system
>> > limits as those of the root cgroup.  If the system is over the global
>> > limit than all cgroups are eligible for writeback.  In this situation
>> > the current code does not distinguish between cgroups over or under
>> > their dirty background limit.
>> >
>> > Vivek Goyal <vgoyal@xxxxxxxxxx> writes:
>> > > If yes, then from design point of view it is little odd that as long
>> > > as we are below background limit, we share the bdi between different
>> > > cgroups. The moment we are above background limit, we fall back to
>> > > algorithm of sharing the disk among individual inodes and forget
>> > > about memory cgroups. Kind of awkward.
>> > >
>> > > This kind of cgroup writeback I think will atleast not solve the problem
>> > > for CFQ IO controller, as we fall back to old ways of writting back inodes
>> > > the moment we cross dirty ratio.
>> >
>> > It might make more sense to reverse the order of the checks in the
>> > proposed over_bground_thresh(): the new version would first check if any
>> > memcg are over limit; assuming none are over limit, then check global
>> > limits.  Assuming that the system is over its background limit and some
>> > cgroups are also over their limits, then the over limit cgroups would
>> > first be written possibly getting the system below its limit.  Does this
>> > address your concern?
>>
>> Do you treat root group also as any other cgroup? If no, then above logic
>> can lead to issue of starvation of root group inode. Or unfair writeback.
>> So I guess it will be important to treat root group same as other groups.
>>
>
> As far as I can say, you should not place programs onto ROOT cgroups if you need
> performance isolation.

Agreed.

> From the code, I think if the system hits dirty_ratio, "1" bit of bitmap should be
> set and background writeback can work for ROOT cgroup seamlessly.
>
> Thanks,
> -Kame

Not quite.  The proposed patches do not set the "1" bit (css_id of
root is 1).  mem_cgroup_balance_dirty_pages() (from patch 10/12)
introduces the following balancing loop:
+       /* balance entire ancestry of current's mem. */
+       for (; mem_cgroup_has_dirty_limit(mem); mem =
parent_mem_cgroup(mem)) {

The loop terminates when mem_cgroup_has_dirty_limit() is called for
the root cgroup.  The bitmap is set in the body of the loop.  So the
root cgroup's bit (bit 1) will never be set in the bitmap.  However, I
think the effect is the same.  The proposed changes in this patch
(11/12) have background writeback first checking if the system is over
limit and if yes, then b_dirty inodes from any cgroup written.  This
means that a small system background limit with an over-{fg or
bg}-limit cgroup could cause other cgroups that are not over their
limit to have their inodes written back.  In an system-over-limit
situation normal system-wide bdi writeback is used (writing inodes in
b_dirty order).  For those who want isolation, a simple rule to avoid
this is to ensure that that sum of all cgroup background_limits is
less than the system background limit.
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