From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH -v8][RFC] mutex: implement adaptive spinning Date: Mon, 12 Jan 2009 18:13:08 +0100 Message-ID: <1231780388.4371.185.camel@laptop> References: <1231774622.4371.96.camel@laptop> <496B6C23.8000808@redhat.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: Linus Torvalds , Ingo Molnar , "Paul E. McKenney" , Gregory Haskins , Matthew Wilcox , Andi Kleen , Chris Mason , Andrew Morton , Linux Kernel Mailing List , linux-fsdevel , linux-btrfs , Thomas Gleixner , Nick Piggin , Peter Morreale , Sven Dietrich , Dmitry Adamushko To: Avi Kivity Return-path: In-Reply-To: <496B6C23.8000808@redhat.com> Sender: linux-btrfs-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Mon, 2009-01-12 at 18:13 +0200, Avi Kivity wrote: > One thing that worries me here is that the spinners will spin on a > memory location in struct mutex, which means that the cacheline holding > the mutex (which is likely to be under write activity from the owner) > will be continuously shared by the spinners, slowing the owner down when > it needs to unshare it. One way out of this is to spin on a location in > struct mutex_waiter, and have the mutex owner touch it when it schedules > out. Yeah, that is what pure MCS locks do -- however I don't think its a feasible strategy for this spin/sleep hybrid. > So: > - each task_struct has an array of currently owned mutexes, appended to > by mutex_lock() That's not going to fly I think. Lockdep does this but its very expensive and has some issues. We're currently at 48 max owners, and still some code paths manage to exceed that. > - mutex waiters spin on mutex_waiter.wait, which they initialize to zero > - when switching out of a task, walk the mutex list, and for each mutex, > bump each waiter's wait variable, and clear the owner array Which is O(n). > - when unlocking a mutex, bump the nearest waiter's wait variable, and > remove from the owner array > > Something similar might be done to spinlocks to reduce cacheline > contention from spinners and the owner. Spinlocks can use 'pure' MCS locks.