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Date: Sat, 23 Nov 2013 05:42:07 +0000 From: "Hart, Darren" <darren.hart@...el.com> To: Davidlohr Bueso <davidlohr@...com> CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "mingo@...nel.org" <mingo@...nel.org>, "peterz@...radead.org" <peterz@...radead.org>, "tglx@...utronix.de" <tglx@...utronix.de>, "efault@....de" <efault@....de>, "jeffm@...e.com" <jeffm@...e.com>, "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>, "scott.norton@...com" <scott.norton@...com>, "tom.vaden@...com" <tom.vaden@...com>, "aswin@...com" <aswin@...com>, "Waiman.Long@...com" <Waiman.Long@...com>, "jason.low2@...com" <jason.low2@...com> Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup On Fri, 2013-11-22 at 21:40 -0800, Darren Hart wrote: > On Fri, 2013-11-22 at 16:56 -0800, Davidlohr Bueso 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. This comes > > at the smaller cost of doing some atomic operations to keep track of > > the list's size. Specifically, increment the counter when an element is > > added to the list, and decrement when it is removed. Of course, if the > > counter is 0, then there are no tasks blocked on a futex. Some special > > considerations: > > > > - increment the counter at queue_lock() as we always end up calling > > queue_me() which adds the element to the list. Upon any error, > > queue_unlock() is called for housekeeping, for which we decrement > > to mach the increment done in queue_lock(). > > > > - decrement the counter at __unqueue_me() to reflect when an element is > > removed from the queue for wakeup related purposes. > > What is the problem you are trying to solve here? Apologies, too quick on the trigger. I see plenty of detail in 0/5. Will spend some time reviewing that. -- Darren Hart Intel Open Source Technology Center Yocto Project - Linux Kernel
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