From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0FE19C7EE29 for ; Fri, 9 Jun 2023 12:37:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240167AbjFIMhn (ORCPT ); Fri, 9 Jun 2023 08:37:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46564 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231612AbjFIMhl (ORCPT ); Fri, 9 Jun 2023 08:37:41 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 00F7A1A2; Fri, 9 Jun 2023 05:37:39 -0700 (PDT) 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 04D55AB6; Fri, 9 Jun 2023 05:38:25 -0700 (PDT) Received: from FVFF77S0Q05N.cambridge.arm.com (FVFF77S0Q05N.cambridge.arm.com [10.1.38.153]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BEE0D3F6C4; Fri, 9 Jun 2023 05:37:37 -0700 (PDT) Date: Fri, 9 Jun 2023 13:37:32 +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 V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE Message-ID: References: <20230531040428.501523-1-anshuman.khandual@arm.com> <20230531040428.501523-7-anshuman.khandual@arm.com> <006be03c-43da-6482-6bc4-83fe65dcd706@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <006be03c-43da-6482-6bc4-83fe65dcd706@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 09, 2023 at 10:00:09AM +0530, Anshuman Khandual wrote: > >> +static u64 branch_type_to_brbcr(int branch_type) > >> +{ > >> + u64 brbcr = BRBCR_EL1_DEFAULT_TS; > >> + > >> + /* > >> + * BRBE need not be paused on PMU interrupt while tracing only > >> + * the user space, bcause it will automatically be inside the > >> + * prohibited region. But even after PMU overflow occurs, the > >> + * interrupt could still take much more cycles, before it can > >> + * be taken and by that time BRBE will have been overwritten. > >> + * Let's enable pause on PMU interrupt mechanism even for user > >> + * only traces. > >> + */ > >> + brbcr |= BRBCR_EL1_FZP; > > I think this is trying to say that we *should* use FZP when sampling the > > kernel (due to IRQ latency), and *can* safely use it when sampling userspace, > > so it would be good to explain it that way around. > > Agreed, following updated comment explains why we should enable FZP > when sampling kernel, otherwise BRBE will capture unwanted records. > It also explains why we should enable FZP even when sampling user > space due to IRQ latency. > > /* > * BRBE should be paused on PMU interrupt while tracing kernel > * space to stop capturing further branch records. Otherwise > * interrupt handler branch records might get into the samples > * which is not desired. > * > * BRBE need not be paused on PMU interrupt while tracing only > * the user space, because it will automatically be inside the > * prohibited region. But even after PMU overflow occurs, the > * interrupt could still take much more cycles, before it can > * be taken and by that time BRBE will have been overwritten. > * Hence enable pause on PMU interrupt mechanism even for user > * only traces as well. > */ > brbcr |= BRBCR_EL1_FZP; Thanks; I think that's a lot clearer! Mark.