From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DB783C433FE for ; Wed, 23 Mar 2022 13:18:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244199AbiCWNT2 (ORCPT ); Wed, 23 Mar 2022 09:19:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45160 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236516AbiCWNT0 (ORCPT ); Wed, 23 Mar 2022 09:19:26 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9929B5FF14; Wed, 23 Mar 2022 06:17:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=tidbQ28V1Z4b3XDUjQz0tCiyzqh7PCycYcQY6xRAP6g=; b=AIBQqFsjXdlLO6yz8HkbcHmlzH Vm/jeFeUAPe6zjc4fBCDgllGCAuRPIlk+vPzP83AflrwTGJRrNsExxI0OnVmsVXNqlMURjfnIMD0k YIL9ic9InO5Bdvh75c48J6Y0MdYfr4x5cHhNEHW+D+sUwne62Eu06Z0YrlpAEfMQoBVp7ixLdchRK HbKkUz6oawwOZ5TdCpAjdUufRxfqTmLGZh0wmOsOF8GqDkZUYkUxojITmvvw4XADpD3tWGSinUEmk Z0Qz1zScJFryDBZkK+pRtDmIhi6zqpJBHUiAu8WPDlvrdtTNdGcfUyTkPXgHqRKeAFatl2cVxLFt7 NaujgeRQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nX0rw-00CYa6-UO; Wed, 23 Mar 2022 13:17:45 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id 6AD0C986200; Wed, 23 Mar 2022 14:17:44 +0100 (CET) Date: Wed, 23 Mar 2022 14:17:44 +0100 From: Peter Zijlstra To: Chengming Zhou Cc: mingo@redhat.com, acme@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, namhyung@kernel.org, eranian@google.com, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, duanxiongchun@bytedance.com, songmuchun@bytedance.com Subject: Re: [External] Re: [PATCH v2 2/6] perf/core: Introduce percpu perf_cgroup Message-ID: <20220323131744.GY8939@worktop.programming.kicks-ass.net> References: <20220322120834.98637-1-zhouchengming@bytedance.com> <20220322120834.98637-3-zhouchengming@bytedance.com> <20220323125116.GX8939@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 23, 2022 at 09:07:01PM +0800, Chengming Zhou wrote: > On 2022/3/23 8:51 下午, Peter Zijlstra wrote: > > On Tue, Mar 22, 2022 at 08:08:30PM +0800, Chengming Zhou wrote: > > > >> diff --git a/kernel/events/core.c b/kernel/events/core.c > >> index 8b5cf2aedfe6..848a3bfa9513 100644 > >> --- a/kernel/events/core.c > >> +++ b/kernel/events/core.c > > > >> @@ -843,11 +845,21 @@ static void perf_cgroup_switch(struct task_struct *task) > >> */ > >> local_irq_save(flags); > >> > >> + cgrp = perf_cgroup_from_task(task, NULL); > >> + if (cgrp == __this_cpu_read(cpu_perf_cgroup)) > >> + goto out; So this compares the cpu thing against the task thing, if matching, we bail. > >> + > >> + __this_cpu_write(cpu_perf_cgroup, cgrp); Then we set cpu thing. > >> + > >> list = this_cpu_ptr(&cgrp_cpuctx_list); > >> list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) { > >> WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0); > >> > >> perf_ctx_lock(cpuctx, cpuctx->task_ctx); > >> + > >> + if (cpuctx->cgrp == cgrp) > >> + continue; > >> + > >> perf_pmu_disable(cpuctx->ctx.pmu); > >> > >> cpu_ctx_sched_out(cpuctx, EVENT_ALL); > >> + cpuctx->cgrp = cgrp But here we already have exactly the same pattern, we match cpuctx thing against task thing and skip/set etc. > > Also, I really don't see the point of cpu_perf_cgroup, cpuctx->cgrp > > should be able to do this. > > But the problem is that we have two cpuctx on the percpu list, do you > mean we should use perf_cgroup of the first cpuctx to compare with > the next task's perf_cgroup ? > > Or we should delete the cgrp in cpuctx, and use this new percpu cpu_perf_cgroup? I'm a bit confused, per the above, you already do exactly what the new cpu_perf_cgroup does on the cpuctx->cgrp variable. AFAICT the only think the new per-cpu variable does is avoid a lock, howveer: > > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -833,6 +833,7 @@ static DEFINE_PER_CPU(struct list_head, > > */ > > static void perf_cgroup_switch(struct task_struct *task) > > { > > + struct perf_cgroup *cgrp; > > struct perf_cpu_context *cpuctx, *tmp; > > struct list_head *list; > > unsigned long flags; > > @@ -843,11 +844,20 @@ static void perf_cgroup_switch(struct ta > > */ > > local_irq_save(flags); > > > > + cgrp = perf_cgroup_from_task(task, NULL); > > + > > list = this_cpu_ptr(&cgrp_cpuctx_list); > > list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) { > > WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0); > > > > + if (READ_ONCE(cpuctx->cgrp == cgrp)) > > + continue I think we can avoid that by doing an early check, hmm? > > + > > perf_ctx_lock(cpuctx, cpuctx->task_ctx); > > + > > + if (cpuctx->cgrp == cgrp) > > + goto next; > > + > > perf_pmu_disable(cpuctx->ctx.pmu); > > > > cpu_ctx_sched_out(cpuctx, EVENT_ALL); > > @@ -855,50 +865,22 @@ static void perf_cgroup_switch(struct ta > > * must not be done before ctxswout due > > * to event_filter_match() in event_sched_out() > > */ > > - cpuctx->cgrp = perf_cgroup_from_task(task, > > - &cpuctx->ctx); > > + WRITE_ONCE(cpuctx->cgrp, cgrp);