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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5ED98C433F5 for ; Tue, 12 Oct 2021 08:47:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 419B561050 for ; Tue, 12 Oct 2021 08:47:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235486AbhJLIte (ORCPT ); Tue, 12 Oct 2021 04:49:34 -0400 Received: from foss.arm.com ([217.140.110.172]:56644 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235462AbhJLItc (ORCPT ); Tue, 12 Oct 2021 04:49:32 -0400 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 86EEAED1; Tue, 12 Oct 2021 01:47:30 -0700 (PDT) Received: from [10.57.21.6] (unknown [10.57.21.6]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A23433F694; Tue, 12 Oct 2021 01:47:28 -0700 (PDT) Subject: Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback To: Will Deacon , German Gomez Cc: Leo Yan , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, John Garry , Mathieu Poirier , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Mike Leach , linux-arm-kernel@lists.infradead.org, coresight@lists.linaro.org References: <20210916154635.1525-1-german.gomez@arm.com> <20210916154635.1525-4-german.gomez@arm.com> <20210923135016.GG400258@leoy-ThinkPad-X240s> <20210923144048.GB603008@leoy-ThinkPad-X240s> <1c6a3a73-27dc-6673-7fe7-34bc7fcb0a68@arm.com> <20211004122724.GC174271@leoy-ThinkPad-X240s> <6b092f13-832f-5d1d-a504-aea96c81bf17@arm.com> <20211006095124.GC14400@leoy-ThinkPad-X240s> <377b54ef-b9c0-9cfc-ef0c-0187d7c493cc@arm.com> <20211012081948.GA5156@willie-the-truck> From: James Clark Message-ID: <55f8978f-e656-5d5f-94e2-f4be4a70656d@arm.com> Date: Tue, 12 Oct 2021 09:47:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20211012081948.GA5156@willie-the-truck> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/10/2021 09:19, Will Deacon wrote: > On Mon, Oct 11, 2021 at 04:55:37PM +0100, German Gomez wrote: >> Hi Leo, >> >> On 06/10/2021 10:51, Leo Yan wrote: >>> On Wed, Oct 06, 2021 at 10:35:20AM +0100, German Gomez wrote: >>> >>> [...] >>> >>>>> So simply say, I think the head pointer monotonically increasing is >>>>> the right thing to do in Arm SPE driver. >>>> I will talk to James about how we can proceed on this. >>> Thanks! >> >> I took this offline with James and, though it looks possible to patch >> the SPE driver to have a monotonically increasing head pointer in order >> to simplify the handling in the perf tool, it could be a breaking change >> for users of the perf_event_open syscall that currently rely on the way >> it works now. >> >> An alternative way we considered to simplify the patch is to change the >> logic inside the find_snapshot callback so that it records the entire >> contents of the aux buffer every time. >> >> What do you think? > > What does intel-pt do? Intel-pt has a wrapped head, which is why it has the intel_pt_find_snapshot() function in perf to try to not save any zeros from the buffer that haven't been written yet. (With a wrapped head pointer it's impossible to tell). Coresight has a monotonically increasing head pointer so it is possible to tell. Recently, Leo removed the Coresight version of find_snapshot() for this reason. It would be nice to do the same for SPE because that function has a heuristic and is also slow, but I imagine that not returning wrapped head pointers could break anything that expects them. James > > Will >