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[68.147.8.254]) by smtp.gmail.com with ESMTPSA id q4sm2874788pgj.44.2020.10.23.13.37.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2020 13:37:31 -0700 (PDT) Date: Fri, 23 Oct 2020 14:37:29 -0600 From: Mathieu Poirier To: Peter Zijlstra Cc: Suzuki Poulose , Sai Prakash Ranjan , Mike Leach , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , coresight@lists.linaro.org, Stephen Boyd , linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf() Message-ID: <20201023203729.GA819775@xps15> References: <20201022212033.GA646497@xps15> <20201023073905.GM2611@hirez.programming.kicks-ass.net> <174e6461-4d46-cb65-c094-c06ee3b21568@arm.com> <20201023094115.GR2611@hirez.programming.kicks-ass.net> <20201023105431.GM2594@hirez.programming.kicks-ass.net> <2457de8f-8bc3-b350-fdc7-61276da31ce6@arm.com> <20201023131628.GY2628@hirez.programming.kicks-ass.net> <728fd89c-78f2-0c5c-0443-c91c62b02f0e@arm.com> <20201023134416.GA2628@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201023134416.GA2628@hirez.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-arm-msm@vger.kernel.org On Fri, Oct 23, 2020 at 03:44:16PM +0200, Peter Zijlstra wrote: > On Fri, Oct 23, 2020 at 02:29:54PM +0100, Suzuki Poulose wrote: > > On 10/23/20 2:16 PM, Peter Zijlstra wrote: > > > On Fri, Oct 23, 2020 at 01:56:47PM +0100, Suzuki Poulose wrote: > > > > > That way another session could use the same sink if it is free. i.e > > > > > > > > perf record -e cs_etm/@sink0/u --per-thread app1 > > > > > > > > and > > > > > > > > perf record -e cs_etm/@sink0/u --per-thread app2 > > > > > > > > both can work as long as the sink is not used by the other session. > > > > > > Like said above, if sink is shared between CPUs, that's going to be a > > > trainwreck :/ Why do you want that? > > > > That ship has sailed. That is how the current generation of systems are, > > unfortunately. But as I said, this is changing and there are guidelines > > in place to avoid these kind of topologies. With the future > > technologies, this will be completely gone. > > I understand that the hardware is like that, but why do you want to > support this insanity in software? > > If you only allow a single sink user (group) at the same time, your > problem goes away. Simply disallow the above scenario, do not allow > concurrent sink users if sinks are shared like this. > > Have the perf-record of app2 above fail because the sink is in-user > already. I agree with you that --per-thread scenarios are easy to deal with, but to support cpu-wide scenarios events must share a sink (because there is one event per CPU). CPU-wide support can't be removed because it has been around for close to a couple of years and heavily used. I also think using the pid of the process that created the events, i.e perf, is a good idea. We just need to agree on how to gain access to it. In Sai's patch you objected to the following: > + struct task_struct *task = READ_ONCE(event->owner); > + > + if (!task || is_kernel_event(event)) Would it be better to use task_nr_pid(current) instead of event->owner? The end result will be exactly the same. There is also no need to check the validity of @current since it is a user process. Thanks, Mathieu [1]. https://elixir.bootlin.com/linux/latest/source/kernel/events/core.c#L6170 > > Only if the hardware has per-CPU sinks can you allow this.