From: Davidlohr Bueso <davidlohr@hp.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Darren Hart <dvhart@linux.intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Mike Galbraith <efault@gmx.de>, Jeff Mahoney <jeffm@suse.com>,
Jason Low <jason.low2@hp.com>, Waiman Long <Waiman.Long@hp.com>,
Tom Vaden <tom.vaden@hp.com>,
"Norton, Scott J" <scott.norton@hp.com>,
"Chandramouleeswaran, Aswin" <aswin@hp.com>
Subject: Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup
Date: Thu, 02 Jan 2014 12:59:17 -0800 [thread overview]
Message-ID: <1388696357.11119.10.camel@buesod1.americas.hpqcorp.net> (raw)
In-Reply-To: <CA+55aFwNPgzORGyKaziqBGqY9Uubuuo0w=rGQvcHS-Ftz4HQ9g@mail.gmail.com>
On Thu, 2014-01-02 at 11:23 -0800, Linus Torvalds wrote:
> On Thu, Jan 2, 2014 at 7:05 AM, Davidlohr Bueso <davidlohr@hp.com> wrote:
> >
> > In futex_wake() there is clearly no point in taking the hb->lock if we know
> > beforehand that there are no tasks to be woken.
>
> Btw, I think we could optimize this a bit further for the wakeup case.
>
> wake_futex() does a get_task_struct(p)/put_task_struct(p) around its
> actual waking logic, and I don't think that's necessary. The task
> structures are RCU-delayed, and the task cannot go away until the
> "q->lock_ptr = NULL" afaik, so you could replace that atomic inc/dec
> with just a RCU read region.
I had originally explored making the whole plist thing more rcu aware
but never got to anything worth sharing. What you say does make a lot of
sense, however, I haven't been able to see any actual improvements. It
doesn't hurt however, so I'd have no problem adding such patch to the
lot.
>
> Maybe it's not a big deal ("wake_up_state()" ends up getting the task
> struct pi_lock anyway, so it's not like we can avoid toucing the task
> structure), but I'm getting the feeling that we're doing a lot of
> unnecessary work here.
I passed this idea through my wakeup measuring program and didn't notice
hardly any difference, just noise, even for large amounts of futexes.
I believe that peterz's idea of lockless batch wakeups is the next step
worth looking into for futexes -- even though the spurious wakeup
problem can become a real pain.
Thanks,
Davidlohr
next prev parent reply other threads:[~2014-01-02 20:59 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-02 15:05 [PATCH v5 0/4] futex: Wakeup optimizations Davidlohr Bueso
2014-01-02 15:05 ` [PATCH v5 1/4] futex: Misc cleanups Davidlohr Bueso
2014-01-11 6:43 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 2/4] futex: Larger hash table Davidlohr Bueso
2014-01-11 7:37 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 3/4] futex: Document ordering guarantees Davidlohr Bueso
2014-01-06 18:58 ` Darren Hart
2014-01-11 7:40 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup Davidlohr Bueso
2014-01-02 19:23 ` Linus Torvalds
2014-01-02 20:59 ` Davidlohr Bueso [this message]
2014-01-06 20:56 ` Darren Hart
2014-01-06 20:52 ` Darren Hart
2014-01-07 3:29 ` Davidlohr Bueso
2014-01-07 17:40 ` Darren Hart
2014-01-11 9:49 ` Paul E. McKenney
2014-01-11 9:52 ` Paul E. McKenney
2014-01-11 18:21 ` Davidlohr Bueso
2014-01-06 0:59 ` [PATCH v5 0/4] futex: Wakeup optimizations Davidlohr Bueso
2014-01-06 1:38 ` [PATCH 5/4] futex: silence uninitialized warnings Davidlohr Bueso
2014-01-06 18:48 ` Darren Hart
2014-01-07 2:55 ` Linus Torvalds
2014-01-07 3:02 ` Davidlohr Bueso
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