linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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Date:	Tue, 24 Sep 2013 09:49:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Tue, Sep 24, 2013 at 06:03:59PM +0200, Oleg Nesterov wrote:
> On 09/24, Peter Zijlstra wrote:
> >
> > +static inline void get_online_cpus(void)
> > +{
> > +	might_sleep();
> > +
> > +	if (current->cpuhp_ref++) {
> > +		barrier();
> > +		return;
> 
> I don't undestand this barrier()... we are going to return if we already
> hold the lock, do we really need it?
> 
> The same for put_online_cpus().

The barrier() is needed because of the possibility of inlining, right?

> > +void __get_online_cpus(void)
> >  {
> > -	if (cpu_hotplug.active_writer == current)
> > +	if (cpuhp_writer_task == current)
> >  		return;
> 
> Probably it would be better to simply inc/dec ->cpuhp_ref in
> cpu_hotplug_begin/end and remove this check here and in
> __put_online_cpus().
> 
> This also means that the writer doing get/put_online_cpus() will
> always use the fast path, and __cpuhp_writer can go away,
> cpuhp_writer_task != NULL can be used instead.

I would need to see the code for this change to be sure.  ;-)

> > +     atomic_inc(&cpuhp_waitcount);
> > +
> > +     /*
> > +      * 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_wq, !__cpuhp_writer);
> > +     preempt_disable();
> > +
> > +     /*
> > +      * It would be possible for cpu_hotplug_done() to complete before
> > +      * the atomic_inc() above; in which case there is no writer waiting
> > +      * and doing a wakeup would be BAD (tm).
> > +      *
> > +      * If however we still observe cpuhp_writer_task here we know
> > +      * cpu_hotplug_done() is currently stuck waiting for cpuhp_waitcount.
> > +      */
> > +     if (atomic_dec_and_test(&cpuhp_waitcount) && cpuhp_writer_task)
> > +             cpuhp_writer_wake();
> 
> cpuhp_writer_wake() here and in __put_online_cpus() looks racy...
> Not only cpuhp_writer_wake() can hit cpuhp_writer_task == NULL (we need
> something like ACCESS_ONCE()), its task_struct can be already freed/reused
> if the writer exits.
> 
> And I don't really understand the logic... This slow path succeds without
> incrementing any counter (except current->cpuhp_ref)? How the next writer
> can notice the fact it should wait for this reader?
> 
> >  void cpu_hotplug_done(void)
> >  {
> > -	cpu_hotplug.active_writer = NULL;
> > -	mutex_unlock(&cpu_hotplug.lock);
> > +	/* Signal the writer is done */
> > +	cpuhp_writer = 0;
> > +	wake_up_all(&cpuhp_wq);
> > +
> > +	/* Wait for any pending readers to be running */
> > +	cpuhp_writer_wait(!atomic_read(&cpuhp_waitcount));
> > +	cpuhp_writer_task = NULL;
> 
> We also need to ensure that the next reader should see all changes
> done by the writer, iow this lacks "realease" semantics.

Good point -- I was expecting wake_up_all() to provide the release
semantics, but code could be reordered into __wake_up()'s critical
section, especially in the case where there was nothing to wake
up, but where there were new readers starting concurrently with
cpu_hotplug_done().

> But, Peter, the main question is, why this is better than
> percpu_rw_semaphore performance-wise? (Assuming we add
> task_struct->cpuhp_ref).
> 
> If the writer is pending, percpu_down_read() does
> 
> 	down_read(&brw->rw_sem);
> 	atomic_inc(&brw->slow_read_ctr);
> 	__up_read(&brw->rw_sem);
> 
> is it really much worse than wait_event + atomic_dec_and_test?
> 
> And! please note that with your implementation the new readers will
> be likely blocked while the writer sleeps in synchronize_sched().
> This doesn't happen with percpu_rw_semaphore.

							Thanx, Paul

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