Re: [PATCH 3/5] cfq-iosched: Allow sync noidle workloads to preempt each other

[Tejun Heo]

On Thu, Jan 07, 2016 at 04:28:14PM +0100, Jan Kara wrote:
> From: Jan Kara <jack@xxxxxxxx>
> 
> The original idea with preemption of sync noidle queues (introduced in
> commit 718eee0579b8 "cfq-iosched: fairness for sync no-idle queues") was
> that we service all sync noidle queues together, we don't idle on any of
> the queues individually and we idle only if there is no sync noidle
> queue to be served. This intention also matches the original test:
> 
> 	if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD
> 	   && new_cfqq->service_tree == cfqq->service_tree)
> 		return true;
> 
> However since at that time cfqq->service_tree was not set for idling
> queues, this test was unreliable and was replaced in commit e4a229196a7c
> "cfq-iosched: fix no-idle preemption logic" by:
> 
> 	if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD &&
> 	    cfqq_type(new_cfqq) == SYNC_NOIDLE_WORKLOAD &&
> 	    new_cfqq->service_tree->count == 1)
> 		return true;
> 
> That was a reliable test but was actually doing something different -
> now we preempt sync noidle queue only if the new queue is the only one
> busy in the service tree.
> 
> These days cfq queue is kept in service tree even if it is idling and
> thus the original check would be safe again. But since we actually check
> that cfq queues are in the same cgroup, of the same priority class and
> workload type (sync noidle), we know that new_cfqq is fine to preempt
> cfqq. So just remove the service tree check.
> 
> Signed-off-by: Jan Kara <jack@xxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun