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[97.120.168.235]) by smtp.gmail.com with ESMTPSA id d24-v6sm18601636pfe.40.2018.11.02.15.13.38 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 02 Nov 2018 15:13:38 -0700 (PDT) Received: by breakout.internal.digitalocean.com (Postfix, from userid 1000) id 2DC1B8A2A45; Fri, 2 Nov 2018 15:13:37 -0700 (PDT) Date: Fri, 2 Nov 2018 15:13:37 -0700 From: Nishanth Aravamudan To: Peter Zijlstra Cc: Jan =?iso-8859-1?Q?H=2E_Sch=F6nherr?= , Ingo Molnar , linux-kernel@vger.kernel.org, Paul Turner , Vincent Guittot , Morten Rasmussen , Tim Chen Subject: Re: [RFC 00/60] Coscheduling for Linux Message-ID: <20181102221337.GA22399@breakout> References: <20180907214047.26914-1-jschoenh@amazon.de> <20180914111251.GC24106@hirez.programming.kicks-ass.net> <1d86f497-9fef-0b19-50d6-d46ef1c0bffa@amazon.de> <20180917113315.GS24124@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180917113315.GS24124@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 17.09.2018 [13:33:15 +0200], Peter Zijlstra wrote: > On Fri, Sep 14, 2018 at 06:25:44PM +0200, Jan H. Schönherr wrote: > > On 09/14/2018 01:12 PM, Peter Zijlstra wrote: > > > On Fri, Sep 07, 2018 at 11:39:47PM +0200, Jan H. Schönherr wrote: > > > >> B) Why would I want this? > > > > > >> In the L1TF context, it prevents other applications from > > >> loading additional data into the L1 cache, while one > > >> application tries to leak data. > > > > > > That is the whole and only reason you did this; > > It really isn't. But as your mind seems made up, I'm not going to > > bother to argue. > > > >> D) What can I *not* do with this? > > >> --------------------------------- > > >> > > >> Besides the missing load-balancing within coscheduled > > >> task-groups, this implementation has the following properties, > > >> which might be considered short-comings. > > >> > > >> This particular implementation focuses on SCHED_OTHER tasks > > >> managed by CFS and allows coscheduling them. Interrupts as well > > >> as tasks in higher scheduling classes are currently out-of-scope: > > >> they are assumed to be negligible interruptions as far as > > >> coscheduling is concerned and they do *not* cause a preemption of > > >> a whole group. This implementation could be extended to cover > > >> higher scheduling classes. Interrupts, however, are an orthogonal > > >> issue. > > >> > > >> The collective context switch from one coscheduled set of tasks > > >> to another -- while fast -- is not atomic. If a use-case needs > > >> the absolute guarantee that all tasks of the previous set have > > >> stopped executing before any task of the next set starts > > >> executing, an additional hand-shake/barrier needs to be added. > > > > > > IOW it's completely friggin useless for L1TF. > > > > Do you believe me now, that L1TF is not "the whole and only reason" > > I did this? :D > > You did mention this work first to me in the context of L1TF, so I might > have jumped to conclusions here. > > Also, I have, of course, been looking at (SMT) co-scheduling, > specifically in the context of L1TF, myself. I came up with a vastly > different approach. Tim - where are we on getting some of that posted? > > Note; that even though I wrote much of that code, I don't particularly > like it either :-) Did your approach get posted to LKML? I never saw it I don't think, and I don't see it on lore. Could it be posted as an RFC, even if not suitable for upstreaming yet, just for comparison? Thanks! -Nish