From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757189Ab3GEIQH (ORCPT ); Fri, 5 Jul 2013 04:16:07 -0400 Received: from merlin.infradead.org ([205.233.59.134]:35187 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757162Ab3GEIQE (ORCPT ); Fri, 5 Jul 2013 04:16:04 -0400 Date: Fri, 5 Jul 2013 10:15:16 +0200 From: Peter Zijlstra To: "Yan, Zheng" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, eranian@google.com, andi@firstfloor.org Subject: Re: [PATCH v2 4/7] perf, x86: Save/resotre LBR stack during context switch Message-ID: <20130705081516.GP18898@dyad.programming.kicks-ass.net> References: <1372663387-11754-1-git-send-email-zheng.z.yan@intel.com> <1372663387-11754-5-git-send-email-zheng.z.yan@intel.com> <20130704124536.GK23916@twins.programming.kicks-ass.net> <51D65B58.1050201@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <51D65B58.1050201@intel.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 05, 2013 at 01:36:24PM +0800, Yan, Zheng wrote: > On 07/04/2013 08:45 PM, Peter Zijlstra wrote: > > On Mon, Jul 01, 2013 at 03:23:04PM +0800, Yan, Zheng wrote: > > > >> @@ -2488,25 +2508,31 @@ static void perf_branch_stack_sched_in(struct task_struct *prev, > >> > >> list_for_each_entry_rcu(pmu, &pmus, entry) { > >> cpuctx = this_cpu_ptr(pmu->pmu_cpu_context); > >> + task_ctx = cpuctx->task_ctx; > >> > >> /* > >> - * check if the context has at least one > >> - * event using PERF_SAMPLE_BRANCH_STACK > >> + * force flush the branch stack if there are cpu-wide events > >> + * using PERF_SAMPLE_BRANCH_STACK > >> + * > >> + * save/restore the branch stack if the task context has > >> + * at least one event using PERF_SAMPLE_BRANCH_STACK > >> */ > >> - if (cpuctx->ctx.nr_branch_stack > 0 > >> - && pmu->flush_branch_stack) { > >> - > >> + bool force_flush = cpuctx->ctx.nr_branch_stack > 0; > >> + if (pmu->branch_stack_sched && > >> + (force_flush || > >> + (task_ctx && task_ctx->nr_branch_stack > 0))) { > >> pmu = cpuctx->ctx.pmu; > >> > >> - perf_ctx_lock(cpuctx, cpuctx->task_ctx); > >> + perf_ctx_lock(cpuctx, task_ctx); > >> > >> perf_pmu_disable(pmu); > >> > >> - pmu->flush_branch_stack(); > >> + pmu->branch_stack_sched(task_ctx, > >> + sched_in, force_flush); > >> > >> perf_pmu_enable(pmu); > >> > >> - perf_ctx_unlock(cpuctx, cpuctx->task_ctx); > >> + perf_ctx_unlock(cpuctx, task_ctx); > >> } > >> } > >> > > > > I never really like this; and yes I know I wrote part of that. Is there > > any way we can get rid of this and to it 'properly' through the events > > that get scheduled? > > > > After all; the LBR usage is through the events, so scheduling the events > > should also manage the LBR state. > > > > What is missing for that to work? > > > > the LBR is shared resource, can be used by multiple events at the same time. Yeah so? There's tons of shared resources in the PMU already. > Strictly speaking，LBR is associated with task, not event. Wrong!, it _is_ associated with events. Events is all there is. Event can be associated with tasks, but that's completely irrelevant. > One example is > there are 5 events using the LBR stack feature, but there are only 4 counters. > So these events need schedule. Saving/restoring LBR on the basis of event is > clearly wrong. Different scheduling and you're wrong. Look at perf_rotate_context(), we'd disable everything at perf_pmu_disable() and enable the entire thing at perf_pmu_enable(), on both sides we'd have the LBR running.