linux-kernel - Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup

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Date:	Thu, 2 Jan 2014 11:23:05 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	Jason Low <jason.low2@...com>,
	Waiman Long <Waiman.Long@...com>, Tom Vaden <tom.vaden@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>
Subject: Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup

On Thu, Jan 2, 2014 at 7:05 AM, Davidlohr Bueso <davidlohr@...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.

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.

This only triggers for the actual case of having a task to wake up,
though, so not your particular test load.

              Linus
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