From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752921AbdHNK5c (ORCPT ); Mon, 14 Aug 2017 06:57:32 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:34932 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752706AbdHNK5b (ORCPT ); Mon, 14 Aug 2017 06:57:31 -0400 Date: Mon, 14 Aug 2017 11:57:29 +0100 From: Will Deacon To: Peter Zijlstra Cc: Mark Rutland , Vince Weaver , linux-kernel@vger.kernel.org, Ingo Molnar , Paul Turner , Mathieu Desnoyers Subject: Re: perf: multiple mmap of fd behavior on x86/ARM Message-ID: <20170814105729.GA14337@arm.com> References: <20170811100127.GB12985@leverpostej> <20170811105252.zuaihhfyvvpvm3uz@hirez.programming.kicks-ass.net> <20170811110639.GC12985@leverpostej> <20170811145330.gtgx3ujmrn6izoka@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170811145330.gtgx3ujmrn6izoka@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 11, 2017 at 04:53:30PM +0200, Peter Zijlstra wrote: > On Fri, Aug 11, 2017 at 12:06:39PM +0100, Mark Rutland wrote: > > On Fri, Aug 11, 2017 at 12:52:52PM +0200, Peter Zijlstra wrote: > > > On Fri, Aug 11, 2017 at 11:01:27AM +0100, Mark Rutland wrote: > > > > On Thu, Aug 10, 2017 at 02:48:52PM -0400, Vince Weaver wrote: > > > > > > > > > > So I was working on my perf_event_tests on ARM/ARM64 (the end goal was to > > > > > get ARM64 rdpmc support working, but apparently those patches never made > > > > > it upstream?) > > > > > > > > IIUC by 'rdpmc' you mean direct userspace counter access? > > > > > > > > Patches for that never made it upstream. Last I saw, there were no > > > > patches in a suitable state for review. > > > > > > > > There are also difficulties (e.g. big.LITTLE systems where the number of > > > > counters can differ across CPUs) which have yet to be solved. > > > > > > How would that be a problem? The API gives an explicit index to use with > > > the 'rdpmc' instruction. > > > > It's a problem because access to unimplemented counters trap. So if a > > task gets migrated from a CPU with N counters to one with N-1, accessing > > counter N would be problematic. > > > > So we'd need to account for that somehow, in addition to the usual > > sequence counter fun to verify the index was valid when the access was > > performed. > > Aah, you need restartable-sequences :-) Or, in the absence of those, I wouldn't mind only supporting this for non-big/little platforms initially. Will