Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759447Ab2HIAwD (ORCPT ); Wed, 8 Aug 2012 20:52:03 -0400 Received: from mail-lpp01m010-f46.google.com ([209.85.215.46]:51492 "EHLO mail-lpp01m010-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758675Ab2HIAwA (ORCPT ); Wed, 8 Aug 2012 20:52:00 -0400 MIME-Version: 1.0 Date: Thu, 9 Aug 2012 02:51:58 +0200 Message-ID: Subject: [BUG] perf: sharing of cpuctx between core and ibs PMU causes problems From: Stephane Eranian To: LKML Cc: Peter Zijlstra , Robert Richter , mingo@elte.hu, "Yan, Zheng" Content-Type: text/plain; charset=UTF-8 X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1258 Lines: 36 Hi, I ran into a problem on my AMD box whereby I would hit the WARN_ON_ONCE(cpuctx->cgrp) in perf_cgroup_switch(). It took me a while to track this down. It turns out that the list_for_each_entry_rcu() loop had multiple iterations. That's normal, we have CPU PMU and IBS PMU. But what caused the warning to fire is that both the core and IBS PMU were pointing to the same cpuctx struct. Thus, the cpuctx->cgrp was already set in the second iteration. Is the warning a false positive? In perf_pmu_register(), there is a search for a matching pmu->task_ctx_nr. Given that the field is pointing to perf_hw_context for both cpu and IBS PMU, there is a match and therefore the cpuctx are shared. The question is: why do we have to share the cpuctx? Note that the same issue probably exists with the Intel uncore PMU. If we need to share, then the perf_cgroup_switch() code needs to change because, as it stands, it is doing the switching twice in this case. Either way something looks wrong here. Any idea? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/