Messages in this thread Patch in this message | | | Subject | Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE | From | Cheng Chao <> | Date | Tue, 13 Sep 2016 12:03:05 +0800 |
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Peter, thank you.
on 09/12/2016 07:41 PM, Peter Zijlstra wrote: > On Mon, Sep 12, 2016 at 01:37:27PM +0200, Peter Zijlstra wrote: >> So what you're saying is that migration_stop_cpu() doesn't work because >> wait_for_completion() dequeues the task. >> >> True I suppose. Not sure I like your solution, nor your implementation >> of the solution much though. >> >> I would much prefer an unconditional cond_resched() there, but also, I >> think we should do what __migrate_swap_task() does, and set wake_cpu. >> >> So something like so.. >> >> --- >> kernel/sched/core.c | 8 ++++++-- >> 1 file changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index ddd5f48551f1..ade772aa9610 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -1063,8 +1063,12 @@ static int migration_cpu_stop(void *data) >> * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because >> * we're holding p->pi_lock. >> */ >> - if (task_rq(p) == rq && task_on_rq_queued(p)) >> - rq = __migrate_task(rq, p, arg->dest_cpu); >> + if (task_rq(p) == rq) { >> + if (task_on_rq_queued(p)) >> + rq = __migrate_task(rq, p, arg->dest_cpu); >> + else >> + p->wake_cpu = arg->dest_cpu; >> + } >> raw_spin_unlock(&rq->lock); >> raw_spin_unlock(&p->pi_lock); >> > > And this, too narrow a constraint do git diff made it go away. >
yes, set wake_cpu is better when try_to_wake_up(). Peter, Is it as a new patch?
> --- > kernel/stop_machine.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c > index ae6f41fb9cba..637798d6b554 100644 > --- a/kernel/stop_machine.c > +++ b/kernel/stop_machine.c > @@ -121,6 +121,11 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg) > cpu_stop_init_done(&done, 1); > if (!cpu_stop_queue_work(cpu, &work)) > return -ENOENT; > + /* > + * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup > + * by doing a preemption. > + */ > + cond_resched(); > wait_for_completion(&done.completion); > return done.ret; > } >
I agree to use cond_resched(). https://lkml.org/lkml/2016/9/12/1228
for CONFIG_PREEMPT=y and CONFIG_PREEMPT_VOLUNTARY=y, this patch seems unnecessary. 1. when CONFIG_PREEMPT=y stop_one_cpu() ->cpu_stop_queue_work() ->spin_unlock_irqrestore() (preempt_enable() calls __preempt_schedule())
2. when CONFIG_PREEMPT_VOLUNTARY=y stop_one_cpu() ->wait_for_completion() ->... ->might_sleep() (calls _cond_resched()
so we really don't need "if defined(CONFIG_PREEMPT_NONE)"?
I also think without the "if defined(CONFIG_PREEMPT_NONE)", the code is more clean,and the logic is still ok.
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index 4a1ca5f..87464a2 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -126,6 +126,15 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg) cpu_stop_init_done(&done, 1); if (!cpu_stop_queue_work(cpu, &work)) return -ENOENT; + +#if defined(CONFIG_PREEMPT_NONE) + /* + * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup + * by doing a preemption. + */ + cond_resched(); +#endif + wait_for_completion(&done.completion); return done.ret; }
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