Commit d1ae8ffd authored Dec 01, 2010 by Vivek Goyal Committed by Jens Axboe Dec 01, 2010

blk-throttle: Trim/adjust slice_end once a bio has been dispatched



o During some testing I did following and noticed throttling stops working.

        - Put a very low limit on a cgroup, say 1 byte per second.
        - Start some reads, this will set slice_end to a very high value.
        - Change the limit to higher value say 1MB/s
        - Now IO unthrottles and finishes as expected.
        - Try to do the read again but IO is not limited to 1MB/s as expected.

o What is happening.
        - Initially low value of limit sets slice_end to a very high value.
        - During updation of limit, slice_end is not being truncated.
        - Very high value of slice_end leads to keeping the existing slice
          valid for a very long time and new slice does not start.
        - tg_may_dispatch() is called in blk_throtle_bio(), and trim_slice()
          is not called in this path. So slice_start is some old value and
          practically we are able to do huge amount of IO.

o There are many ways it can be fixed. I have fixed it by trying to
  adjust/cleanup slice_end in trim_slice(). Generally we extend slices if bio
  is big and can't be dispatched in one slice. After dispatch of bio, readjust
  the slice_end to make sure we don't end up with huge values.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

parent 54787556

block/blk-throttle.c

+16 −0

Original line number	Diff line number	Diff line
		@@ -355,6 +355,12 @@ throtl_start_new_slice(struct throtl_data td, struct throtl_grp tg, bool rw)
		tg->slice_end[rw], jiffies);
		}

		static inline void throtl_set_slice_end(struct throtl_data *td,
		struct throtl_grp *tg, bool rw, unsigned long jiffy_end)
		{
		tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
		}

		static inline void throtl_extend_slice(struct throtl_data *td,
		struct throtl_grp *tg, bool rw, unsigned long jiffy_end)
		{
		@@ -391,6 +397,16 @@ throtl_trim_slice(struct throtl_data td, struct throtl_grp tg, bool rw)
		if (throtl_slice_used(td, tg, rw))
		return;

		/*
		* A bio has been dispatched. Also adjust slice_end. It might happen
		* that initially cgroup limit was very low resulting in high
		* slice_end, but later limit was bumped up and bio was dispached
		* sooner, then we need to reduce slice_end. A high bogus slice_end
		* is bad because it does not allow new slice to start.
		*/

		throtl_set_slice_end(td, tg, rw, jiffies + throtl_slice);

		time_elapsed = jiffies - tg->slice_start[rw];

		nr_slices = time_elapsed / throtl_slice;