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Wed, 02 Dec 2020 22:42:01 -0800 (PST) Date: Thu, 3 Dec 2020 14:41:57 +0800 From: Leo Yan To: Will Deacon , Mathieu Poirier Subject: Re: [PATCH] drivers/perf: Enable PID_IN_CONTEXTIDR with SPE Message-ID: <20201203064157.GF28939@leoy-ThinkPad-X240s> References: <20201130162454.28255-1-james.clark@arm.com> <20201130164650.GA25187@willie-the-truck> <20201201041040.GC28939@leoy-ThinkPad-X240s> <20201201230935.GD28496@willie-the-truck> <20201203063941.GE28939@leoy-ThinkPad-X240s> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201203063941.GE28939@leoy-ThinkPad-X240s> User-Agent: Mutt/1.9.4 (2018-02-28) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201203_014205_508689_7D89C134 X-CRM114-Status: GOOD ( 35.39 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Al Grant , Suzuki K Poulose , John Garry , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, James Clark , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Looping in Mathieu at this time. On Thu, Dec 03, 2020 at 02:39:41PM +0800, Leo Yan wrote: > Hi Will, > > [ + Mathieu ] > > On Tue, Dec 01, 2020 at 11:09:36PM +0000, Will Deacon wrote: > > On Tue, Dec 01, 2020 at 12:10:40PM +0800, Leo Yan wrote: > > > On Mon, Nov 30, 2020 at 04:46:51PM +0000, Will Deacon wrote: > > > > On Mon, Nov 30, 2020 at 06:24:54PM +0200, James Clark wrote: > > > > > Enable PID_IN_CONTEXTIDR by default when Arm SPE is enabled. > > > > > This flag is required to get PID data in the SPE trace. Without > > > > > it the perf tool will report 0 for PID which isn't very useful, > > > > > especially when doing system wide profiling or profiling > > > > > applications that fork. > > > > > > > > Can perf not figure out the pid some other way? (e.g. by tracing context > > > > switches and correlating that with the SPE data?). > > > > > > For perf 'per-thread' mode, we can use context switch trace event as > > > assisted info to select thread context. But for "system wide" mode and > > > "snapshot" mode in perf tool, since the trace data is continuous, I > > > think we cannot use context switch trace event to correlate the SPE > > > trace data. > > > > Is there no way to correlate them with something like CNTVCT? > > Good point. Yes, we can convert CNTVCT to system time; I read the > code in the perf's intel-pt.c and found the timestamp is used to > correlate the auxtrace heap. I think it's better to dig more for > detailed implementation. > > > > > Also, how does this work with pid namespaces? > > > > > > Here we are studying the implemetation of Intel-PT and Arm CoreSight. > > > > > > The context ID is stored into the hardware trace data when record; > > > afterwards when perf tool decodes the trace data and detects the > > > packet for context ID, it will select the machine's thread context in > > > perf [1]. Since the perf tool gathers all the threads infomation in > > > perf data file, based on the context ID, it can find the corresponding > > > thread pointer with function machine__find_thread() [2]. > > > > > > Since your question is for "pid namespace", to be honest, I don't know > > > how perf tool to handle any confliction for differrent processes share > > > the same PID, and I am not sure if you are asking CGroup related stuff > > > or not. If this cannot answer your question, please let me know. > > > > My point was that the pid value written to CONTEXTIDR is a global pid > > and does not take namespacing into account. If perf is run inside a pid > > namespace, it will therefore not work. > > Understand now. > > The perf events PERF_RECORD_ITRACE_START/PERF_RECORD_SWITCH/ > PERF_RECORD_SWITCH_CPU_WIDE can be used to set pid/tid in perf. So this > would be a safe way for perf tool running in pid namespace. > > Loop in Mathieu, this is a common issue for both Arm SPE and CoreSight > (IIRC, though CoreSight's timestamp is not strictly attaching to Arm arch > timer counter, the trend is to unify this for using arch timer > counter). > > I think James could continue to upstream a new patch by following your > suggestion for enabling PID_IN_CONTEXTIDR, eventually, it's a feature > for Arm SPE to record CONTEXTIDR in its packet. > > Your questions inspired me, thanks! > > Leo _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel