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SubjectRe: [PATCH] blk-throttle: Reset group slice when limits are changed
On Tue, Mar 15, 2011 at 01:54:56PM -0400, Vivek Goyal wrote:
> Lina reported that if throttle limits are initially very high and then
> dropped, then no new bio might be dispatched for a long time. And the
> reason being that after dropping the limits we don't reset the existing
> slice and do the rate calculation with new low rate and account the bios
> dispatched at high rate. To fix it, reset the slice upon rate change.

Hi Jens,

Can you please apply this patch too.

Thanks
Vivek

>
> https://lkml.org/lkml/2011/3/10/298
>
> Another problem with very high limit is that we never queued the
> bio on throtl service tree. That means we kept on extending the
> group slice but never trimmed it. Fix that also by regulary
> trimming the slice even if bio is not being queued up.
>
> Reported-by: Lina Lu <lulina_nuaa@foxmail.com>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
> block/blk-throttle.c | 25 ++++++++++++++++++++++++-
> 1 file changed, 24 insertions(+), 1 deletion(-)
>
> Index: linux-2.6-block/block/blk-throttle.c
> ===================================================================
> --- linux-2.6-block.orig/block/blk-throttle.c 2011-03-15 13:37:04.122389034 -0400
> +++ linux-2.6-block/block/blk-throttle.c 2011-03-15 13:37:26.328370086 -0400
> @@ -756,6 +756,15 @@ static void throtl_process_limit_change(
> " riops=%u wiops=%u", tg->bps[READ], tg->bps[WRITE],
> tg->iops[READ], tg->iops[WRITE]);
>
> + /*
> + * Restart the slices for both READ and WRITES. It
> + * might happen that a group's limit are dropped
> + * suddenly and we don't want to account recently
> + * dispatched IO with new low rate
> + */
> + throtl_start_new_slice(td, tg, 0);
> + throtl_start_new_slice(td, tg, 1);
> +
> if (throtl_tg_on_rr(tg))
> tg_update_disptime(td, tg);
> }
> @@ -821,7 +830,8 @@ throtl_schedule_delayed_work(struct thro
>
> struct delayed_work *dwork = &td->throtl_work;
>
> - if (total_nr_queued(td) > 0) {
> + /* schedule work if limits changed even if no bio is queued */
> + if (total_nr_queued(td) > 0 || td->limits_changed) {
> /*
> * We might have a work scheduled to be executed in future.
> * Cancel that and schedule a new one.
> @@ -1002,6 +1012,19 @@ int blk_throtl_bio(struct request_queue
> /* Bio is with-in rate limit of group */
> if (tg_may_dispatch(td, tg, bio, NULL)) {
> throtl_charge_bio(tg, bio);
> +
> + /*
> + * We need to trim slice even when bios are not being queued
> + * otherwise it might happen that a bio is not queued for
> + * a long time and slice keeps on extending and trim is not
> + * called for a long time. Now if limits are reduced suddenly
> + * we take into account all the IO dispatched so far at new
> + * low rate and * newly queued IO gets a really long dispatch
> + * time.
> + *
> + * So keep on trimming slice even if bio is not queued.
> + */
> + throtl_trim_slice(td, tg, rw);
> goto out;
> }
>


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