Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH 8/8] tick/nohz: Kick only _queued_ task whose tick dependency is updated | Date | Thu, 22 Apr 2021 14:01:58 +0200 |
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From: Marcelo Tosatti <mtosatti@redhat.com>
When the tick dependency of a task is updated, we want it to aknowledge the new state and restart the tick if needed. If the task is not running, we don't need to kick it because it will observe the new dependency upon scheduling in. But if the task is running, we may need to send an IPI to it so that it gets notified.
Unfortunately we don't have the means to check if a task is running in a race free way. Checking p->on_cpu in a synchronized way against p->tick_dep_mask would imply adding a full barrier between prepare_task_switch() and tick_nohz_task_switch(), which we want to avoid in this fast-path.
Therefore we blindly fire an IPI to the task's CPU.
Meanwhile we can check if the task is queued on the CPU rq because p->on_rq is always set to TASK_ON_RQ_QUEUED _before_ schedule() and its full barrier that precedes tick_nohz_task_switch(). And if the task is queued on a nohz_full CPU, it also has fair chances to be running as the isolation constraints prescribe running single tasks on full dynticks CPUs.
So use this as a trick to check if we can spare an IPI toward a non-running task.
NOTE: For the ordering to be correct, it is assumed that we never deactivate a task while it is running, the only exception being the task deactivating itself while scheduling out.
Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com> Cc: Yunfeng Ye <yeyunfeng@huawei.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> --- include/linux/sched.h | 2 ++ kernel/sched/core.c | 5 +++++ kernel/time/tick-sched.c | 19 +++++++++++++++++-- 3 files changed, 24 insertions(+), 2 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h index ef00bb22164c..64dd6f698f3a 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1999,6 +1999,8 @@ static inline void set_task_cpu(struct task_struct *p, unsigned int cpu) #endif /* CONFIG_SMP */ +extern bool sched_task_on_rq(struct task_struct *p); + /* * In order to reduce various lock holder preemption latencies provide an * interface to see if a vCPU is currently running or not. diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 98191218d891..08526227d200 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1580,6 +1580,11 @@ static inline void uclamp_post_fork(struct task_struct *p) { } static inline void init_uclamp(void) { } #endif /* CONFIG_UCLAMP_TASK */ +bool sched_task_on_rq(struct task_struct *p) +{ + return task_on_rq_queued(p); +} + static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags) { if (!(flags & ENQUEUE_NOCLOCK)) diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index ad5c3905196a..faba7881048f 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -324,8 +324,6 @@ void tick_nohz_full_kick_cpu(int cpu) static void tick_nohz_kick_task(struct task_struct *tsk) { - int cpu = task_cpu(tsk); - /* * If the task concurrently migrates to another cpu, * we guarantee it sees the new tick dependency upon @@ -340,6 +338,23 @@ static void tick_nohz_kick_task(struct task_struct *tsk) * tick_nohz_task_switch() smp_mb() (atomic_fetch_or()) * LOAD p->tick_dep_mask LOAD p->cpu */ + int cpu = task_cpu(tsk); + + /* + * If the task is not running, run_posix_cpu_timers + * has nothing to elapsed, can spare IPI in that + * case. + * + * activate_task() STORE p->tick_dep_mask + * STORE p->on_rq + * __schedule() (switch to task 'p') smp_mb() (atomic_fetch_or()) + * LOCK rq->lock LOAD p->on_rq + * smp_mb__after_spin_lock() + * tick_nohz_task_switch() + * LOAD p->tick_dep_mask + */ + if (!sched_task_on_rq(tsk)) + return; preempt_disable(); if (cpu_online(cpu)) -- 2.25.1
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