From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752296AbcHLKCO (ORCPT ); Fri, 12 Aug 2016 06:02:14 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42420 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751529AbcHLKBa (ORCPT ); Fri, 12 Aug 2016 06:01:30 -0400 Date: Fri, 12 Aug 2016 12:01:23 +0200 From: Jiri Olsa To: Mark Rutland Cc: linux-kernel@vger.kernel.org, acme@kernel.org, adrian.hunter@intel.com, alexander.shishkin@linux.intel.com, hekuang@huawei.com, jolsa@kernel.org, mingo@redhat.com, peterz@infradead.org, wangnan0@huawei.com Subject: Re: [RFCv3 2/2] perf: util: support sysfs supported_cpumask file Message-ID: <20160812100123.GB25813@krava> References: <1470933366-1364-1-git-send-email-mark.rutland@arm.com> <1470933366-1364-3-git-send-email-mark.rutland@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1470933366-1364-3-git-send-email-mark.rutland@arm.com> User-Agent: Mutt/1.6.2 (2016-07-01) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Fri, 12 Aug 2016 10:01:30 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 11, 2016 at 05:36:06PM +0100, Mark Rutland wrote: > The perf tools can read a cpumask file for a PMU, describing a subset of > CPUs which that PMU covers. So far this has only been used to cater for > uncore PMUs, which in practice happen to only have a single CPU > described in the mask. > > Until recently, the perf tools only correctly handled cpumask containing > a single CPU, and only when monitoring in system-wide mode. For example, > prior to commit 00e727bb389359c8 ("perf stat: Balance opening and > reading events"), a mask with more than a single CPU could cause > perf stat to hang. When a CPU PMU covers a subset of CPUs, but lacks a > cpumask, perf record will fail to open events (on the cores the PMU does > not support), and gives up. > > For systems with heterogeneous CPUs such as ARM big.LITTLE systems, this > presents a problem. We have a PMU for each microarchitecture (e.g. a big > PMU and a little PMU), and would like to expose a cpumask for each (so > as to allow perf record and other tools to do the right thing). However, > doing so kernel-side will cause old perf binaries to not function (e.g. > hitting the issue solved by 00e727bb389359c8), and thus commits the > cardinal sin of breaking (existing) userspace. > > To address this chicken-and-egg problem, this patch adds support got a > new file, supported_cpumask, which is largely identical to the existing > cpumask file. A kernel can expose this file, knowing that new perf > binaries will correctly support it, while old perf binaries will not > look for it (and thus will not be broken). I might have asked before, but what's the kernel side state of this? thanks, jirka