From: Linus Torvalds <torvalds@linux-foundation.org>
To: Davidlohr Bueso <davidlohr@hp.com>
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, 2 Jan 2014 11:23:05 -0800 [thread overview]
Message-ID: <CA+55aFwNPgzORGyKaziqBGqY9Uubuuo0w=rGQvcHS-Ftz4HQ9g@mail.gmail.com> (raw)
In-Reply-To: <1388675120-8017-5-git-send-email-davidlohr@hp.com>
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.
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
next prev parent reply other threads:[~2014-01-02 19:23 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 [this message]
2014-01-02 20:59 ` Davidlohr Bueso
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CA+55aFwNPgzORGyKaziqBGqY9Uubuuo0w=rGQvcHS-Ftz4HQ9g@mail.gmail.com' \
--to=torvalds@linux-foundation.org \
--cc=Waiman.Long@hp.com \
--cc=aswin@hp.com \
--cc=davidlohr@hp.com \
--cc=dvhart@linux.intel.com \
--cc=efault@gmx.de \
--cc=jason.low2@hp.com \
--cc=jeffm@suse.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=paulmck@linux.vnet.ibm.com \
--cc=peterz@infradead.org \
--cc=scott.norton@hp.com \
--cc=tglx@linutronix.de \
--cc=tom.vaden@hp.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).