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Fri, 01 May 2020 16:29:28 +0100 MIME-Version: 1.0 Date: Fri, 01 May 2020 16:29:27 +0100 From: Marc Zyngier To: Leo Yan Subject: Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer In-Reply-To: <20200501151448.GA9650@leoy-ThinkPad-X240s> References: <20200320093545.28227-1-leo.yan@linaro.org> <20200430145823.GA25258@willie-the-truck> <4d924f705245c797a19d3a73eb0c1ba0@kernel.org> <20200430160436.GC13575@hirez.programming.kicks-ass.net> <20200430161815.GE25258@willie-the-truck> <20200501151448.GA9650@leoy-ThinkPad-X240s> User-Agent: Roundcube Webmail/1.4.3 Message-ID: <0a74ba5e82a41dca0a4156f2039e6be6@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: leo.yan@linaro.org, will@kernel.org, peterz@infradead.org, mark.rutland@arm.com, mingo@redhat.com, acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, catalin.marinas@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, mathieu.poirier@linaro.org, mike.leach@linaro.org, Al.Grant@arm.com, James.Clark@arm.com, tglx@linutronix.de X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200501_082930_047678_B344CD4B X-CRM114-Status: GOOD ( 22.18 ) 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 , Mathieu Poirier , Peter Zijlstra , Jiri Olsa , linux-kernel@vger.kernel.org, Arnaldo Carvalho de Melo , Alexander Shishkin , Ingo Molnar , James Clark , Catalin Marinas , Namhyung Kim , tglx@linutronix.de, Will Deacon , linux-arm-kernel@lists.infradead.org, Mike Leach Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 2020-05-01 16:14, Leo Yan wrote: > Hi all, > > On Thu, Apr 30, 2020 at 05:18:15PM +0100, Will Deacon wrote: >> On Thu, Apr 30, 2020 at 06:04:36PM +0200, Peter Zijlstra wrote: >> > On Thu, Apr 30, 2020 at 04:29:23PM +0100, Marc Zyngier wrote: >> > >> > > I wonder if we could/should make __sched_clock_offset available even when >> > > CONFIG_HAVE_UNSTABLE_SCHED_CLOCK isn't defined. It feels like it would >> > > help with this particular can or worm... >> > >> > Errrgh. __sched_clock_offset is only needed on x86 because we transition >> > from one clock device to another on boot. It really shouldn't exist on >> > anything sane. >> >> I think we still transition from jiffies on arm64, because we don't >> register >> with sched_clock until the timer driver probes. Marc, is that right? >> >> > Let me try and understand your particular problem better. >> >> I think the long and short of it is that userspace needs a way to >> convert >> the raw counter cycles into a ns value that can be compared against >> values >> coming out of sched_clock. To do this accurately, I think it needs the >> cycles value at the point when sched_clock was initialised. > > Will's understanding is exactly what I want to resolve in this patch. > > The background info is for the ARM SPE [1] decoding with perf tool, if > the timestamp is enabled, it uses the generic timer's counter as > timestamp source. SPE trace data only contains the raw counter cycles, > as Will mentioned, the perf tool needs to convert it to a coordinate > value with sched_clock. This is why this patch tries to calculate the > offset between the raw counter's ns value and sched_clock, eventually > this offset value will be used by SPE's decoding code in Perf tool to > calibrate a 'correct' timestamp. > > Based on your suggestions, I will use __sched_clock_offset to resolve > the accuracy issue in patch v2. (I noticed Peter suggested to use a > new API for wrapping clock_data structure, IIUC, __sched_clock_offset > is more straightforward for this case). I think Peter's point was *not* to use __sched_clock_offset which appears to be only there for the purpose of an x86 workaround (and not available to other architecture), but to instead expose the relevant field (epoch_cyc) to the perf subsystem. I feel it makes sense to make this a generic API, and see whether x86 can move over to it rather than the other way around. Thanks, M. -- Jazz is not dead. It just smells funny... _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel