Messages in this thread Patches in this message | | | Date | Wed, 14 Oct 2020 10:33:21 +0200 | From | Peter Zijlstra <> | Subject | Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task |
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On Tue, Oct 13, 2020 at 02:13:28PM -0300, Marcelo Tosatti wrote:
> > Yes but if the task isn't running, run_posix_cpu_timers() doesn't have > > anything to elapse. So indeed we can spare the IPI if the task is not > > running. Provided ordering makes sure that the task sees the new dependency > > when it schedules in of course. > > True. > > * p->on_cpu <- { 0, 1 }: > * > * is set by prepare_task() and cleared by finish_task() such that it will be > * set before p is scheduled-in and cleared after p is scheduled-out, both > * under rq->lock. Non-zero indicates the task is running on its CPU. > > > CPU-0 (tick_set_dep) CPU-1 (task switch) > > STORE p->tick_dep_mask > smp_mb() (atomic_fetch_or()) > LOAD p->on_cpu > > > context_switch(prev, next) > STORE next->on_cpu = 1 > ... [*] > > LOAD current->tick_dep_mask >
That load is in tick_nohz_task_switch() right? (which BTW is placed completely wrong) You could easily do something like the below I suppose.
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 81632cd5e3b7..2a5fafe66bb0 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -410,6 +410,14 @@ void __tick_nohz_task_switch(void) ts = this_cpu_ptr(&tick_cpu_sched); if (ts->tick_stopped) { + /* + * tick_set_dep() (this) + * + * STORE p->tick_dep_mask STORE p->on_cpu + * smp_mb() smp_mb() + * LOAD p->on_cpu LOAD p->tick_dep_mask + */ + smp_mb(); if (atomic_read(¤t->tick_dep_mask) || atomic_read(¤t->signal->tick_dep_mask)) tick_nohz_full_kick();
re tick_nohz_task_switch() being placed wrong, it should probably be placed before finish_lock_switch(). Something like so.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index cf044580683c..5c92c959824f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4084,6 +4084,7 @@ static struct rq *finish_task_switch(struct task_struct *prev) vtime_task_switch(prev); perf_event_task_sched_in(prev, current); finish_task(prev); + tick_nohz_task_switch(); finish_lock_switch(rq); finish_arch_post_lock_switch(); kcov_finish_switch(current); @@ -4121,7 +4122,6 @@ static struct rq *finish_task_switch(struct task_struct *prev) put_task_struct_rcu_user(prev); } - tick_nohz_task_switch(); return rq; } diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 81632cd5e3b7..c8bddc9fb792 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -402,10 +402,8 @@ void __tick_nohz_task_switch(void) unsigned long flags; struct tick_sched *ts; - local_irq_save(flags); - if (!tick_nohz_full_cpu(smp_processor_id())) - goto out; + return; ts = this_cpu_ptr(&tick_cpu_sched); @@ -414,8 +412,6 @@ void __tick_nohz_task_switch(void) atomic_read(¤t->signal->tick_dep_mask)) tick_nohz_full_kick(); } -out: - local_irq_restore(flags); } /* Get the boot-time nohz CPU list from the kernel parameters. */
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