From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH -v8][RFC] mutex: implement adaptive spinning Date: Mon, 12 Jan 2009 19:23:38 +0200 Message-ID: <496B7C9A.7030108@redhat.com> References: <1231774622.4371.96.camel@laptop> <496B6C23.8000808@redhat.com> <1231780388.4371.185.camel@laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed 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: Peter Zijlstra Return-path: In-Reply-To: <1231780388.4371.185.camel@laptop> Sender: linux-btrfs-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Peter Zijlstra wrote: > 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. > Bummer. >> 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. > Might make it per-cpu instead, and set a bit in the mutex when scheduling out so we know not to remove it from the list on unlock. >> - 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). > It may be better than O(n) cpus banging on the mutex for the lock duration. Of course we should avoid walking the part of the list where non-spinning owners wait (or maybe have a separate list for spinners). >> - 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. > I'll read up on those, thanks. -- error compiling committee.c: too many arguments to function