Messages in this thread | | | Date | Tue, 24 Sep 2013 18:31:46 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() |
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On 09/24, Peter Zijlstra wrote: > > +void __get_online_cpus(void) > { > - if (cpu_hotplug.active_writer == current) > + /* Support reader-in-writer recursion */ > + if (__cpuhp_writer == current) > return; > - mutex_lock(&cpu_hotplug.lock); > > - if (WARN_ON(!cpu_hotplug.refcount)) > - cpu_hotplug.refcount++; /* try to fix things up */ > + atomic_inc(&cpuhp_waitcount); > > - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) > - wake_up_process(cpu_hotplug.active_writer); > - mutex_unlock(&cpu_hotplug.lock); > + /* > + * We either call schedule() in the wait, or we'll fall through > + * and reschedule on the preempt_enable() in get_online_cpus(). > + */ > + preempt_enable_no_resched(); > + wait_event(cpuhp_readers, !__cpuhp_writer); > + preempt_disable(); > + > + if (atomic_dec_and_test(&cpuhp_waitcount)) > + wake_up_all(&cpuhp_writer);
Yes, this should fix the races with the exiting writer, but still this doesn't look right afaics.
In particular let me repeat,
> void cpu_hotplug_begin(void) > { > - cpu_hotplug.active_writer = current; > + unsigned int count = 0; > + int cpu; > + > + lockdep_assert_held(&cpu_add_remove_lock); > > - for (;;) { > - mutex_lock(&cpu_hotplug.lock); > - if (likely(!cpu_hotplug.refcount)) > - break; > - __set_current_state(TASK_UNINTERRUPTIBLE); > - mutex_unlock(&cpu_hotplug.lock); > - schedule(); > + __cpuhp_writer = current; > + > + /* > + * After this everybody will observe writer and take the slow path. > + */ > + synchronize_sched();
synchronize_sched() is slow. The new readers will likely notice __cpuhp_writer != NULL much earlier and they will be blocked in __get_online_cpus() while the writer sleeps before it actually enters the critical section.
Or I completely misunderstood this all?
Oleg.
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