linux-kernel - Re: [PATCH 3/3] blkio: Increment the blkio cgroup stats for real now

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Date:	Mon, 5 Apr 2010 09:53:25 -0700
From:	Divyesh Shah <dpshah@...gle.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	nauman@...gle.com, ctalbott@...gle.com
Subject: Re: [PATCH 3/3] blkio: Increment the blkio cgroup stats for real now

On Mon, Apr 5, 2010 at 8:12 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Fri, Apr 02, 2010 at 04:36:34PM -0700, Divyesh Shah wrote:
>> On Fri, Apr 2, 2010 at 12:10 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
>> > On Thu, Apr 01, 2010 at 03:01:41PM -0700, Divyesh Shah wrote:
>> >> We also add start_time_ns and io_start_time_ns fields to struct request
>> >> here to record the time when a request is created and when it is
>> >> dispatched to device. We use ns uints here as ms and jiffies are
>> >> not very useful for non-rotational media.
>> >>
>> >> Signed-off-by: Divyesh Shah<dpshah@...gle.com>
>> >> ---
>> >>
>> >>  block/blk-cgroup.c     |   60 ++++++++++++++++++++++++++++++++++++++++++++++--
>> >>  block/blk-cgroup.h     |   14 +++++++++--
>> >>  block/blk-core.c       |    6 +++--
>> >>  block/cfq-iosched.c    |    4 ++-
>> >>  include/linux/blkdev.h |   20 +++++++++++++++-
>> >>  5 files changed, 95 insertions(+), 9 deletions(-)
>> >>
>> >> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>> >> index ad6843f..9af7257 100644
>> >> --- a/block/blk-cgroup.c
>> >> +++ b/block/blk-cgroup.c
>> >> @@ -15,6 +15,7 @@
>> >>  #include <linux/kdev_t.h>
>> >>  #include <linux/module.h>
>> >>  #include <linux/err.h>
>> >> +#include <linux/blkdev.h>
>> >>  #include "blk-cgroup.h"
>> >>
>> >>  static DEFINE_SPINLOCK(blkio_list_lock);
>> >> @@ -55,6 +56,26 @@ struct blkio_cgroup *cgroup_to_blkio_cgroup(struct cgroup *cgroup)
>> >>  }
>> >>  EXPORT_SYMBOL_GPL(cgroup_to_blkio_cgroup);
>> >>
>> >> +/*
>> >> + * Add to the appropriate stat variable depending on the request type.
>> >> + * This should be called with the blkg->stats_lock held.
>> >> + */
>> >> +void io_add_stat(uint64_t *stat, uint64_t add, unsigned int flags)
>> >> +{
>> >> +     if (flags & REQ_RW)
>> >> +             stat[IO_WRITE] += add;
>> >> +     else
>> >> +             stat[IO_READ] += add;
>> >> +     /*
>> >> +      * Everywhere in the block layer, an IO is treated as sync if it is a
>> >> +      * read or a SYNC write. We follow the same norm.
>> >> +      */
>> >> +     if (!(flags & REQ_RW) || flags & REQ_RW_SYNC)
>> >> +             stat[IO_SYNC] += add;
>> >> +     else
>> >> +             stat[IO_ASYNC] += add;
>> >> +}
>> >> +
>> >
>> > Hi Divyesh,
>> >
>> > Can we have any request based information limited to cfq and not put that
>> > in blkio-cgroup. The reason being that I am expecting that some kind of
>> > max bw policy interface will not necessarily be implemented at CFQ
>> > level. We might have to implement it at higher level so that it can
>> > work with all dm/md devices. If that's the case, then it might very well
>> > be either a bio based interface also.
>> >
>> > So just keeping that possibility in mind, can we keep blk-cgroup as
>> > generic as possible and not necessarily make it dependent on "struct
>> > request".
>>
>> Ok. I do understand the motivation for keeping the request related
>> info out of blk-cgroup. Everything except the rq->cmd_flags can be
>> easily done away with. Maybe I'll need to have CFQ send the sync and
>> direction bits as args to the functions that need it. Not ideal coz
>> we'll have functions with many args but I guess its not that bad too.
>>
>> >
>> > If you implement, two dimensional arrays for stats then we can have
>> > following function.
>> >
>> > blkio_add_stat(enum stat_type var enum stat_sub_type var_type, u64 val)
>>
>> I would want to avoid calls like these from CFQ into the blkcg code
>> because many CFQ events trigger update for multiple stats (you'll see
>> more with stats in later patchsets) and doing these calls
>> independently for each stat would mean that we would also need to grab
>> the stats_lock multiple times when we could've avoided that.
>
> I understand the need to club the updates and reduce the need of taking
> stats_lock multiple times. I was thinking of any of following.
>
> - Get rid of reset interface per cgroup. Rely on changing ioscheduler on
>  request queue and that will get rid of stats_lock entirely.

This takes away the ability to reset stats at will which is very
useful when debugging and for testing IO controller.

> - Can we use a function blkio_add_stat() with variable number of arguments
>  so that more than one stat can be updated in a single call?

I really don't like this at all.

> If you have other ideas to implement it without assuming "struct rq" in
> blk-cgroup, please do that.

I've already got rid of any rq assumptions in blk-cgroup. The only
place where we're using rq is for rq_start_time_ns() and
rq_io-start_time_ns() functions but they are not used by the
blk-cgroup code directly (only CFQ uses them). For another user of
io-controller, we can implement a bio based functions.

>
> Thanks
> Vivek
>
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