From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 63426C43381 for ; Wed, 6 Mar 2019 08:32:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2CF902064A for ; Wed, 6 Mar 2019 08:32:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="WJHD6U8O" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729160AbfCFIcS (ORCPT ); Wed, 6 Mar 2019 03:32:18 -0500 Received: from merlin.infradead.org ([205.233.59.134]:38708 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728969AbfCFIcR (ORCPT ); Wed, 6 Mar 2019 03:32:17 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=VxEQufSkxfx4daBMXnY7yCKUoh7b79A19vdrUWhMAFY=; b=WJHD6U8OC2C2VsJiPKYKcBJ8t OQv9bxzu5hjf0ZHUKzm52oQ3mLJHfjpvLxI7NP6yDZxANJQy7ODIuJYgS2EG/g04HNXllSRFrGJal wJakq9gKv+ysVIIVva8U1iemmRfj/KduNsR4EvcVDEOjAG19pdNgbzTd83k6v3RFKR21II2FqGRic 3zcN6eVS9hibJbtz0PdGM5Xv8z04jIIOqX5WBGM2L/cDch45jsAKX3tCaohJ7+6BM4pezFOJktlNg bzpGKbUxJTSs1TxP0i/Oz4NBzaZ2pyUNyuNmcUBBYGjm2ww/FOeNRVn5joDMkyPs9zztz+tpP5iDt Dwn+Y1x5g==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1h1Rwn-0007h5-UA; Wed, 06 Mar 2019 08:30:43 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 5915C20288B9E; Wed, 6 Mar 2019 09:30:39 +0100 (CET) Date: Wed, 6 Mar 2019 09:30:39 +0100 From: Peter Zijlstra To: Mathieu Desnoyers Cc: "H.J. Lu" , libc-alpha , Thomas Gleixner , linux-kernel , linux-api , "Paul E . McKenney" , Boqun Feng , Andy Lutomirski , Dave Watson , Paul Turner , Andrew Morton , Russell King , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , Chris Lameter , Ben Maurer , rostedt , Josh Triplett , Linus Torvalds , Catalin Marinas , Will Deacon , Michael Kerrisk , Joel Fernandes , Carlos O'Donell , Florian Weimer Subject: Re: [PATCH for 5.1 0/3] Restartable Sequences updates for 5.1 Message-ID: <20190306083039.GS32477@hirez.programming.kicks-ass.net> References: <20190305194755.2602-1-mathieu.desnoyers@efficios.com> <1689743723.311.1551817115045.JavaMail.zimbra@efficios.com> <20190305215848.GQ32477@hirez.programming.kicks-ass.net> <486623963.509.1551825130539.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <486623963.509.1551825130539.JavaMail.zimbra@efficios.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 05, 2019 at 05:32:10PM -0500, Mathieu Desnoyers wrote: > >> * Adaptative mutex improvements > >> > >> I have done a prototype using rseq to implement an adaptative mutex which > >> can detect preemption using a rseq critical section. This ensures the > >> thread doesn't continue to busy-loop after it returns from preemption, and > >> calls sys_futex() instead. This is part of a user-space prototype branch [2], > >> and does not require any kernel change. > > > > I'm still not convinced that is actually the right way to go about > > things. The kernel heuristic is spin while the _owner_ runs, and we > > don't get preempted, obviously. > > > > And the only userspace spinning that makes sense is to cover the cost of > > the syscall. Now Obviously PTI wrecked everything, but before that > > syscalls were actually plenty fast and you didn't need many cmpxchg > > cycles to amortize the syscall itself -- which could then do kernel side > > adaptive spinning (when required). > > Indeed with PTI the system calls are back to their slow self. ;) > > You point about owner is interesting. Perhaps there is one tweak that I > should add in there. We could write the owner thread ID in the lock word. This is already required for PI (and I think robust) futexes. There have been proposals for FUTEX_LOCK and FUTEX_UNLOCK (!PI) primitives that require the same. Waiman had some patches; but I think all went under because 'important' stuff happened. > When trying to grab a lock, one of a few situations can happen: > > - It's unlocked, so we grab it by storing our thread ID, > - It's locked, and we can fetch the CPU number of the thread owning it > if we can access its (struct rseq *)->cpu_id through a lookup using its > thread ID, We can then check whether it's the same CPU we are running on. That might just work with threads (private futexes; which are the majority these these I think), but will obviously not work with regular (shared) futexes. > - If so, we _know_ we should let the owner run, so we call futex right away, > no spinning. We can even boost it for priority inheritance mutexes, > - If it's owned by a thread which was last running on a different CPU, > then it may make sense to actively try to grab the lock by spinning > up to a certain number of loops (which can be either fixed or adaptative). > After that limit, call futex. If preempted while looping, call futex. > > Do you see this as an improvement over what exists today, or am I > on the wrong track ? That's probably better than what they have today. Last time I looked at libc pthread I got really sad -- arguably that was a long time ago, and some of that stuff is because POSIX, but still. Some day we should redesign all that.. futex2 etc.