On Wed 01-03-17 10:07:44, Hou Tao wrote:
> When adding a cfq_group into the cfq service tree, we use CFQ_IDLE_DELAY
> as the delay of cfq_group's vdisktime if there have been other cfq_groups
> already.
>
> When cfq is under iops mode, commit 9a7f38c42c2b ("cfq-iosched: Convert
> from jiffies to nanoseconds") could result in a large iops delay and
> lead to an abnormal io schedule delay for the added cfq_group. To fix
> it, we just need to revert to the old CFQ_IDLE_DELAY value: HZ / 5
> when iops mode is enabled.
>
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.8+
> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
OK, I agree my commit broke the logic in this case. Thanks for the fix.
Please add also tag:
Fixes: 9a7f38c42c2b92391d9dabaf9f51df7cfe5608e4
I somewhat disagree with the fix though. See below:
> +static inline u64 cfq_get_cfqg_vdisktime_delay(struct cfq_data *cfqd)
> +{
> + if (!iops_mode(cfqd))
> + return CFQ_IDLE_DELAY;
> + else
> + return nsecs_to_jiffies64(CFQ_IDLE_DELAY);
> +}
> +
So using nsecs_to_jiffies64(CFQ_IDLE_DELAY) when in iops mode just does not
make any sense. AFAIU the code in cfq_group_notify_queue_add() we just want
to add the cfqg as the last one in the tree. So returning 1 from
cfq_get_cfqg_vdisktime_delay() in iops mode should be fine as well.
Frankly, vdisktime is in fixed-point precision shifted by
CFQ_SERVICE_SHIFT so using CFQ_IDLE_DELAY does not make much sense in any
case and just adding 1 to maximum vdisktime should be fine in all the
cases. But that would require more testing whether I did not miss anything
subtle.
Honza
> static void
> cfq_group_notify_queue_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
> {
> @@ -1380,7 +1388,8 @@ cfq_group_notify_queue_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
> n = rb_last(&st->rb);
> if (n) {
> __cfqg = rb_entry_cfqg(n);
> - cfqg->vdisktime = __cfqg->vdisktime + CFQ_IDLE_DELAY;
> + cfqg->vdisktime = __cfqg->vdisktime +
> + cfq_get_cfqg_vdisktime_delay(cfqd);
> } else
> cfqg->vdisktime = st->min_vdisktime;
> cfq_group_service_tree_add(st, cfqg);
> --
> 2.5.0
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
