nohz: Use IPI implicit full barrier against rq->nr_running r/w (3882ec64) · Commits · 戴 / test

A full dynticks CPU is allowed to stop its tick when a single task runs. Meanwhile when a new task gets enqueued, the CPU must be notified so that it can restart its tick to maintain local fairness and other accounting details. This notification is performed by way of an IPI. Then when the target receives the IPI, we expect it to see the new value of rq->nr_running. Hence the following ordering scenario: CPU 0 CPU 1 write rq->running get IPI smp_wmb() smp_rmb() send IPI read rq->nr_running But Paul Mckenney says that nowadays IPIs imply a full barrier on all architectures. So we can safely remove this pair and rely on the implicit barriers that come along IPI send/receive. Lets just comment on this new assumption. Acked-by:

Peter Zijlstra <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Kevin Hilman <khilman@linaro.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Viresh Kumar <viresh.kumar@linaro.org> Signed-off-by:

Frederic Weisbecker <fweisbec@gmail.com>

kernel/sched/core.c

+5 −4

Original line number	Original line	Diff line number	Diff line
	@@ -740,10 +740,11 @@ bool sched_can_stop_tick(void)

	rq = this_rq();		rq = this_rq();

	/* Make sure rq->nr_running update is visible after the IPI */		/*
	smp_rmb();		* More than one running task need preemption.
			* nr_running update is assumed to be visible
	/* More than one running task need preemption */		* after IPI is sent from wakers.
			*/
	if (rq->nr_running > 1)		if (rq->nr_running > 1)
	return false;		return false;

kernel/sched/sched.h

+8 −2

Original line number	Original line	Diff line number	Diff line
	@@ -1221,8 +1221,14 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
	#ifdef CONFIG_NO_HZ_FULL		#ifdef CONFIG_NO_HZ_FULL
	if (prev_nr < 2 && rq->nr_running >= 2) {		if (prev_nr < 2 && rq->nr_running >= 2) {
	if (tick_nohz_full_cpu(rq->cpu)) {		if (tick_nohz_full_cpu(rq->cpu)) {
	/* Order rq->nr_running write against the IPI */		/*
	smp_wmb();		* Tick is needed if more than one task runs on a CPU.
			* Send the target an IPI to kick it out of nohz mode.
			*
			* We assume that IPI implies full memory barrier and the
			* new value of rq->nr_running is visible on reception
			* from the target.
			*/
	tick_nohz_full_kick_cpu(rq->cpu);		tick_nohz_full_kick_cpu(rq->cpu);
	}		}
	}		}

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