From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753085AbdI1MOB (ORCPT ); Thu, 28 Sep 2017 08:14:01 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:56226 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752525AbdI1MOA (ORCPT ); Thu, 28 Sep 2017 08:14:00 -0400 Date: Thu, 28 Sep 2017 14:13:49 +0200 (CEST) From: Thomas Gleixner To: Peter Zijlstra cc: Gerd Gerats , mingo@redhat.com, dvhart@infradead.org, LKML Subject: Re: [RFC] futex: hashbucket as list of futex instead of waiters In-Reply-To: <20170928105814.cfzsyi42asybctfo@hirez.programming.kicks-ass.net> Message-ID: References: <20170910213324.GA24252@pluto.fritz.box> <20170928105814.cfzsyi42asybctfo@hirez.programming.kicks-ass.net> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) 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 Thu, 28 Sep 2017, Peter Zijlstra wrote: > On Sun, Sep 10, 2017 at 11:41:58PM +0200, Gerd Gerats wrote: > > When using futex as a condition variable, for example: to manage a > > threadpool, there may be a lot of threads inside the futex_wait to sleep on > > this futex. The futex_hash_bucket consists therefore of many struct futex_q > > for the same futex. > > > > On bad luck another futex, used as mutex, hashed into the same bucket. > > Every futex_wake on this mutex, has to scan the whole chain of above waiter > > to find the struct futex_q for this mutex. For non-unusual threadpool sizes > > of more than 20, this should be a considerable effort. > > > > I therefore suggest to include in the hash-bucketchain only one struct > > futex_q per futex and to queue additional waiter in an extrachain at the > > 'top' futex_q entry. Thus different futex are isolated from each other, the > > cost of a hash collision is reduced. > > So I don't dislike that idea.. however > > > To show the idea, I added a sample patch. Here, the plist is exchanged for > > a futex-specific implementation. kernel/pring.h is certainly not not the > > right place. > > So I suppose the purpose of that plist in futex is to enable waking up > the highest prio waiter, but with the advent of SCHED_DEADLINE that no > longer works. > > I think Thomas resisted going the RB-tree route earlier.. The only problem I see with the RB tree is the performance overhead and people are constantly complaining about futex performance anyway. But it might be worth a try. Thanks, tglx