From fd2ac4f4a65a7f34b0bc6433fcca1192d7ba8b8e Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 18 Mar 2014 21:12:53 +0100 Subject: nohz: Use nohz own full kick on 2nd task enqueue Now that we have a nohz full remote kick based on irq work, lets use it to notify a CPU that it's exiting single task mode. This unbloats a bit the scheduler IPI that the nohz code was abusing for its cool "callable anywhere/anytime" properties. Acked-by: Peter Zijlstra Cc: Andrew Morton Cc: Ingo Molnar Cc: Kevin Hilman Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Viresh Kumar Signed-off-by: Frederic Weisbecker --- kernel/sched/sched.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'kernel/sched/sched.h') diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 31cc02ebc54e..599a72aff5ea 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1223,7 +1223,7 @@ static inline void add_nr_running(struct rq *rq, unsigned count) if (tick_nohz_full_cpu(rq->cpu)) { /* Order rq->nr_running write against the IPI */ smp_wmb(); - smp_send_reschedule(rq->cpu); + tick_nohz_full_kick_cpu(rq->cpu); } } #endif -- cgit v1.2.2 From 3882ec643997757824cd5f25180cd8a787b9dbe1 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 18 Mar 2014 22:54:04 +0100 Subject: nohz: Use IPI implicit full barrier against rq->nr_running r/w 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 Cc: Andrew Morton Cc: Ingo Molnar Cc: Kevin Hilman Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Viresh Kumar Signed-off-by: Frederic Weisbecker --- kernel/sched/sched.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) (limited to 'kernel/sched/sched.h') diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 599a72aff5ea..eb8567610295 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1221,8 +1221,14 @@ static inline void add_nr_running(struct rq *rq, unsigned count) #ifdef CONFIG_NO_HZ_FULL if (prev_nr < 2 && rq->nr_running >= 2) { 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); } } -- cgit v1.2.2