From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752500AbcEGItU (ORCPT ); Sat, 7 May 2016 04:49:20 -0400 Received: from www.linutronix.de ([62.245.132.108]:33460 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750936AbcEGItS (ORCPT ); Sat, 7 May 2016 04:49:18 -0400 Date: Sat, 7 May 2016 10:47:38 +0200 (CEST) From: Thomas Gleixner To: Darren Hart cc: LKML , Sebastian Andrzej Siewior , Linus Torvalds , Darren Hart , Peter Zijlstra , Ingo Molnar , Michael Kerrisk , Davidlohr Bueso , Chris Mason , "Carlos O'Donell" , Torvald Riegel , Eric Dumazet Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation In-Reply-To: <20160506181843.GF48432@f23x64.localdomain> Message-ID: References: <20160505204230.932454245@linutronix.de> <20160505204354.051976225@linutronix.de> <20160506181843.GF48432@f23x64.localdomain> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 6 May 2016, Darren Hart wrote: > On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote: > > From: Sebastian Siewior > > > > The per process hash is allocated on the fly at the first futex operation of a > > process. The size of the hash is determined by a system wide default setting > > controlled by the sys admin, This is suboptimal for RT applications and > > applications with pathological futex abuse, > > > > - For RT applications its important to allocate the per process hash before the > > first futex operation to avoid the allocation on the first futex operation. > > > > - For pathological applications which use gazillions of futexes its useful to > > allocate a hash greater than the default hash size. > > "it's" or preferably "it is" > > > > > Add a futex op which allows to preallocate the hash with the requested > > "allows for preallocating" > > > size. The size is limited by the systemwide maximum hash size, which can be > > system-wide > > > set by the admin. The requested size is rounded up to the next order of 2. > > > > The function can be called several times, but ony the first call results in a > > hash allocation of the requested size as there is no non-intrusive way to > > reallocate/rehash in a multithreaded application. > > > > Note, that this call must be issued before the first futex operation in the > > process because that would automatically allocate the default sized hash. > > So this seems like it could be tricky for the user as system libraries, like > glibc, make use of futexes. Can we guarantee that "sys_futex" is not called by > the time main() is called? To the extent of my testing I never observed that the hash was automatically created when I called futex(PREALLOC) right away in main. But yes, that might need some thought. Thanks, tglx