From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760234AbcHaKqd (ORCPT ); Wed, 31 Aug 2016 06:46:33 -0400 Received: from foss.arm.com ([217.140.101.70]:57452 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760199AbcHaKq3 (ORCPT ); Wed, 31 Aug 2016 06:46:29 -0400 Date: Wed, 31 Aug 2016 11:46:16 +0100 From: Mark Rutland To: Jiri Olsa 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: <20160831104415.GD4783@leverpostej> References: <1470933366-1364-1-git-send-email-mark.rutland@arm.com> <1470933366-1364-3-git-send-email-mark.rutland@arm.com> <20160812100123.GB25813@krava> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160812100123.GB25813@krava> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Apologies for the delay in replying. On Fri, Aug 12, 2016 at 12:01:23PM +0200, Jiri Olsa wrote: > 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? Kernel-side, we do not currently expose a cpumask, and I do not have a current patch series to do so. I wanted to figure out if this was the right direction or whether I was going off into the weeds. Clearly that's jsut confusing, so I guess I should respin this long with the kernel-side patches? Implementation wise, it's fairly trivial to add (e.g. [1]). Thanks, Mark. [1] http://lkml.kernel.org/r/1466529109-21715-9-git-send-email-jeremy.linton@arm.com