Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH 3/3] nohz: Use IPI implicit full barrier against rq->nr_running r/w | Date | Wed, 19 Mar 2014 19:28:22 +0100 |
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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 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 comes along IPI send/receive. Lets just comment on this new assumption.
Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Jens Axboe <axboe@fb.com> 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> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> --- kernel/sched/core.c | 9 +++++---- kernel/sched/sched.h | 10 ++++++++-- 2 files changed, 13 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a07c3a4..6d32618 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -666,10 +666,11 @@ bool sched_can_stop_tick(void) rq = this_rq(); - /* Make sure rq->nr_running update is visible after the IPI */ - smp_rmb(); - - /* More than one running task need preemption */ + /* + * More than one running task need preemption. + * nr_running update is assumed to be visible + * after IPI is sent from wakers. + */ if (rq->nr_running > 1) return false; diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 3b165c1..ec248dc 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1231,8 +1231,14 @@ static inline void inc_nr_running(struct rq *rq) #ifdef CONFIG_NO_HZ_FULL if (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); } } -- 1.8.3.1
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