Messages in this thread | | | Date | Mon, 29 Jun 2015 06:02:51 +0200 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and stop_cpus_lock |
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On 06/26, Oleg Nesterov wrote: > > 2 cmpxchg()'s vs 2 spin_lock()'s. Plus wake_up(), but we can check > waitqueue_active(). > > Do you think thi will be noticeably slower? > > Of course, if it races with another stop_two_cpus/stop_cpus it will > sleep, but in this case we need to wait anyway. > > > And I don't think that percpu-rwsem instead of stop_cpu_mutex makes > sense. at least I don't understand how can it help. OK, stop_two_cpus() > can use percpu_down_read() to avoid the deadlock with stop_cpus(), but > you still need double-lock... So I don't think this will make it faster, > this will just penalize stop_cpus(). Or I misunderstood. > > So I am still not convinced... But probably I am too biased ;)
Yes... I'll probably try to make v2, this version is overcomplicated and buggy.
> Btw. I can't understand the cpu_active() checks in stop_two_cpus(). > Do we really need them?
Ah, please ignore.
Yes, we can't rely on stopper->enabled check in cpu_stop_queue_work(), cpu_stop_signal_done() does not update multi_stop_data->num_threads / ->thread_ack. So we need to ensure that cpu_online() == T for both CPUS or multi_cpu_stop() can hang.
But we can't use cpu_online() instead, take_cpu_down() can be already queued.
So this relies on the fact that CPU_DOWN_PREPARE (which removes CPU from cpu_active_mask) is called before stop_machine(take_cpu_down) and we do not care that cpu_active() is not stable; if we see cpu_active() cpu_online() can't change unders us because take_cpu_down() was not queued.
If we change stop_two_cpus() to use stop_work_alloc_one() it can use cpu_online(),
int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg) { struct cpu_stop_work *work1, *work2; struct cpu_stop_done done; struct multi_stop_data msdata = { .fn = fn, .data = arg, .num_threads = 2, .active_cpus = cpumask_of(cpu1), };
set_state(&msdata, MULTI_STOP_PREPARE); cpu_stop_init_done(&done, 2);
if (cpu1 > cpu2) swap(cpu1, cpu2);
work1 = stop_work_alloc_one(cpu1, true); work2 = stop_work_alloc_one(cpu2, true); /* stop_machine() is blocked, cpu can't go away */ if (cpu_online(cpu1) && cpu_online(cpu2)) { work1->fn = work2->fn = multi_cpu_stop; work1->arg = work2->arg = &msdata; work1->done = work2->done = &done;
preempt_disable(); cpu_stop_queue_work(cpu1, work1); cpu_stop_queue_work(cpu2, work2); preempt_enable(); wait_for_completion(&done.completion); }
stop_work_free_one(cpu1); stop_work_free_one(cpu2); stop_work_wake_up();
return done.executed ? done.ret : -ENOENT; }
Oleg.
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