From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 7CF3471B48; Tue, 21 May 2024 13:44:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716299096; cv=none; b=hkAH603DTHaZSg3PVACUvYIMWUIxDDF8rZE2c6D+3SQ4K3f/9c3arUOBaPNuElyeu/t9y7WfQh7SEWV+1POlUOe5ZzovMDuNlxDA3ySp2H2RTHc39HJpgtE3vSrYpfmEoRjHmzsaKGRataHZToewoSLpI6WJllZ0A7M/CxCaQNI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716299096; c=relaxed/simple; bh=H48cizhNNLDxjU+31xIZUJnQnKmSiED2ZFACjcWGzz8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=L9oQofuP5h3u8Q9so1K7nIJ2RsmE4+AmNFr+19SF9OHBGcNPMD06vt8IpWbywDvUVINBpABtyoC/F4whbfUkQ42jZqARMVbl12stc+bx1PZ0E5kPqPdKLvcwY+Vj6i1hb65vShQb/5LK6nD8F23Hg3JubkpVgAVeImQsyIb15so= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id F2DACDA7; Tue, 21 May 2024 06:45:17 -0700 (PDT) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id DB4663F641; Tue, 21 May 2024 06:44:51 -0700 (PDT) Date: Tue, 21 May 2024 14:44:49 +0100 From: Mark Rutland To: Anshuman Khandual Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, will@kernel.org, catalin.marinas@arm.com, Mark Brown , James Clark , Rob Herring , Marc Zyngier , Suzuki Poulose , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , linux-perf-users@vger.kernel.org Subject: Re: [PATCH V17 3/9] drivers: perf: arm_pmu: Add infrastructure for branch stack sampling Message-ID: References: <20240405024639.1179064-1-anshuman.khandual@arm.com> <20240405024639.1179064-4-anshuman.khandual@arm.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240405024639.1179064-4-anshuman.khandual@arm.com> On Fri, Apr 05, 2024 at 08:16:33AM +0530, Anshuman Khandual wrote: > In order to support the Branch Record Buffer Extension (BRBE), we need to > extend the arm_pmu framework with some basic infrastructure for the branch > stack sampling which arm_pmu drivers can opt-in using a new feature flag > called 'has_branch_stack'. Subsequent patches will use this to add support > for BRBE in the PMUv3 driver. Please, just use ther *exact* wording I asked for last time: | In order to support the Branch Record Buffer Extension (BRBE), we need to | extend the arm_pmu framework with some basic infrastructure for branch stack | sampling which arm_pmu drivers can opt-in to using. Subsequent patches will | use this to add support for BRBE in the PMUv3 driver. At this point in the commit message, the 'has_branch_stack' flag doesn't matter, and dropping the 'to' after 'opt-in' makes this painful to read. > Branch stack sampling support i.e capturing branch records during execution > in core perf, rides along with normal HW events being scheduled on the PMU. > This prepares ARMV8 PMU framework for branch stack support on relevant PMUs > with required HW implementation. Please delete this paragraph. > With BRBE, the hardware records branches into a hardware FIFO, which will > be sampled by software when perf events overflow. A task may be context- > switched an arbitrary number of times between overflows, and to avoid > losing samples we need to save the current records when a task is context- > switched out. To do these we'll need to use the pmu::sched_task() callback, > and we'll also need to allocate some per-task storage space via event flag > PERF_ATTACH_TASK_DATA. [...] > /* The events for a given PMU register set. */ > struct pmu_hw_events { > /* > @@ -66,6 +78,17 @@ struct pmu_hw_events { > struct arm_pmu *percpu_pmu; > > int irq; > + > + struct branch_records *branches; > + > + /* Active context for task events */ > + void *branch_context; > + > + /* Active events requesting branch records */ > + unsigned int branch_users; > + > + /* Active branch sample type filters */ > + unsigned long branch_sample_type; > }; At this point in the series I understand why we have the 'branches' and 'branch_users' fields, but the 'branch_context' and 'branch_sample_type' fields haven't been introduced and are not obvious. What exactly is branch_context, and why is that a 'void *' ? I can understand if that's a PMU-specific structure to track the active branch records, but if so I don't understand why 'branch_sample_type' isn't folded into that. Mark.