Commit 80e3d87b authored Dec 12, 2014 by Tim Chen Committed by Ingo Molnar Jan 30, 2015

sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target



This patch adds checks that prevens futile attempts to move rt tasks
to a CPU with active tasks of equal or higher priority.

This reduces run queue lock contention and improves the performance of
a well known OLTP benchmark by 0.7%.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Cc: Suruchi Kadu <suruchi.a.kadu@intel.com>
Cc: Doug Nelson<doug.nelson@intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1421430374.2399.27.camel@schen9-desk2.jf.intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 3847b272

kernel/sched/rt.c

+16 −1

Original line number	Diff line number	Diff line
		@@ -1340,7 +1340,12 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
		curr->prio <= p->prio)) {
		int target = find_lowest_rq(p);

		if (target != -1)
		/*
		* Don't bother moving it if the destination CPU is
		* not running a lower priority task.
		*/
		if (target != -1 &&
		p->prio < cpu_rq(target)->rt.highest_prio.curr)
		cpu = target;
		}
		rcu_read_unlock();
		@@ -1617,6 +1622,16 @@ static struct rq find_lock_lowest_rq(struct task_struct task, struct rq *rq)

		lowest_rq = cpu_rq(cpu);

		if (lowest_rq->rt.highest_prio.curr <= task->prio) {
		/*
		* Target rq has tasks of equal or higher priority,
		* retrying does not release any lock and is unlikely
		* to yield a different result.
		*/
		lowest_rq = NULL;
		break;
		}

		/* if the prio of this runqueue changed, try again */
		if (double_lock_balance(rq, lowest_rq)) {
		/*

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