cfq-iosched: Allow parent cgroup to preempt its child (3984aa55) · Commits · 戴 / test

Currently we don't allow sync workload of one cgroup to preempt sync workload of any other cgroup. This is because we want to achieve service separation between cgroups. However in cases where cgroup preempting is ancestor of the current cgroup, there is no need of separation and idling introduces unnecessary overhead. This hurts for example the case when workload is isolated within a cgroup but journalling threads are in root cgroup. Simple way to demostrate the issue is using: dbench4 -c /usr/share/dbench4/client.txt -t 10 -D /mnt 1 on ext4 filesystem on plain SATA drive (mounted with barrier=0 to make difference more visible). When all processes are in the root cgroup, reported throughput is 153.132 MB/sec. When dbench process gets its own blkio cgroup, reported throughput drops to 26.1006 MB/sec. Fix the problem by making check in cfq_should_preempt() more benevolent and allow preemption by ancestor cgroup. This improves the throughput reported by dbench4 to 48.9106 MB/sec. Acked-by:

Tejun Heo <tj@kernel.org> Signed-off-by:

Jan Kara <jack@suse.com> Signed-off-by:

Jens Axboe <axboe@fb.com>

block/cfq-iosched.c

+18 −1

Original line number	Diff line number	Diff line
		@@ -632,6 +632,13 @@ static inline struct cfq_group cfqg_parent(struct cfq_group cfqg)
		return pblkg ? blkg_to_cfqg(pblkg) : NULL;
		}

		static inline bool cfqg_is_descendant(struct cfq_group *cfqg,
		struct cfq_group *ancestor)
		{
		return cgroup_is_descendant(cfqg_to_blkg(cfqg)->blkcg->css.cgroup,
		cfqg_to_blkg(ancestor)->blkcg->css.cgroup);
		}

		static inline void cfqg_get(struct cfq_group *cfqg)
		{
		return blkg_get(cfqg_to_blkg(cfqg));
		@@ -758,6 +765,11 @@ static void cfqg_stats_xfer_dead(struct cfq_group *cfqg)
		#else /* CONFIG_CFQ_GROUP_IOSCHED */

		static inline struct cfq_group cfqg_parent(struct cfq_group cfqg) { return NULL; }
		static inline bool cfqg_is_descendant(struct cfq_group *cfqg,
		struct cfq_group *ancestor)
		{
		return true;
		}
		static inline void cfqg_get(struct cfq_group *cfqg) { }
		static inline void cfqg_put(struct cfq_group *cfqg) { }

		@@ -3953,7 +3965,12 @@ cfq_should_preempt(struct cfq_data cfqd, struct cfq_queue new_cfqq,
		if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
		return true;

		if (new_cfqq->cfqg != cfqq->cfqg)
		/*
		* Treat ancestors of current cgroup the same way as current cgroup.
		* For anybody else we disallow preemption to guarantee service
		* fairness among cgroups.
		*/
		if (!cfqg_is_descendant(cfqq->cfqg, new_cfqq->cfqg))
		return false;

		if (cfq_slice_used(cfqq))

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