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 X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 13067C433E0 for ; Fri, 5 Mar 2021 15:00:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CCE4E65011 for ; Fri, 5 Mar 2021 15:00:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229955AbhCEO7m (ORCPT ); Fri, 5 Mar 2021 09:59:42 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40692 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229843AbhCEO7L (ORCPT ); Fri, 5 Mar 2021 09:59:11 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 56966C061574 for ; Fri, 5 Mar 2021 06:59:11 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=R/OkZYQVM7XyISLc6jdwXi4U1aEK7k/C0FjZm3dELGM=; b=U6ndaEufEOfSAhtZyRCaH1C3gK RtyLSTEArq5ZWQzUV238E1tmsJpPYg8lE3MJGTFzw8W0AElF3GboT6sX6NylBO1Ds5LIJab/vSASs 1PQTD85Q+I0FxFEblx0H67cWlf4YD/0AcjedzZs6IlpWsfmUalyxVCEfq/dQ2TsU9t7376+6YavUd HUgL2FzCNUMLnMPr53o2fx6rc/xvaf7ARxtt/rwIsj17DZb5qLqO5PgfgCiKSCFJDHSjtMrblH5qH A1v6nqQ9xKXtc2tn9zNKbUZhrRZ0L7H5A6uQVBo0trWSXpYXGdK16n/Z+fdgVX+vv/IpVri/oe/BV kkoYbBCw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lIBtr-00Brah-Le; Fri, 05 Mar 2021 14:58:06 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id AD93D30011C; Fri, 5 Mar 2021 15:57:54 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id A1373203D8A3F; Fri, 5 Mar 2021 15:57:54 +0100 (CET) Date: Fri, 5 Mar 2021 15:57:54 +0100 From: Peter Zijlstra To: Linus Torvalds Cc: Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Linux Kernel Mailing List Subject: Re: "struct perf_sample_data" alignment Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 05, 2021 at 09:36:30AM +0100, Peter Zijlstra wrote: > On Thu, Mar 04, 2021 at 07:45:44PM -0800, Linus Torvalds wrote: > > That ____cacheline_aligned goes back many years, this is not new, it > > seems to come from back in 2014: commit 2565711fb7d7 ("perf: Improve > > the perf_sample_data struct layout"). > > long time ago... > > > But it really seems entirely and utterly bogus. That cacheline > > alignment makes things *worse*, when the variables are on the local > > stack. The local stack is already going to be dirty and in the cache, > > and aligning those things isn't going to - and I quote from the code > > in that commend in that commit - "minimize the cachelines touched". > > > > Quite the reverse. It's just going to make the stack frame use *more* > > memory, and make any cacheline usage _worse_. > > IIRC there is more history here, but I can't seem to find references > just now. > > What I remember is that since perf_sample_data is fairly large, > unconditionally initializing the whole thing is *slow* (and > -fauto-var-init=zero will hurt here). > > So at some point I removed that full initialization and made sure we > only unconditionally touched the first few variables, which gave a > measurable speedup. > > Then things got messy again and the commit 2565711fb7d7 referenced above > was cleanup, to get back to that initial state. > > Now, you're right that __cacheline_aligned on on-stack (and this is > indeed mostly on-stack) is fairly tedious (there were a few patches > recently to reduce the amount of on-stack instances). > > I'll put it on the todo list, along with that hotplug stuff (which I > tried to fix but ended up with an even bigger mess). I suppose we can > try and not have the alignment for the on-stack instances while > preserving it for the few off-stack ones. > > Also; we're running on the NMI stack, and that's not typically hot. This seems to be it... (completely untested) --- diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index 3f7f89ea5e51..918a296d2ca2 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -1032,7 +1032,9 @@ struct perf_sample_data { u64 cgroup; u64 data_page_size; u64 code_page_size; -} ____cacheline_aligned; +}; + +typedef struct perf_sample_data perf_sample_data_t ____cacheline_aligned; /* default value for data source */ #define PERF_MEM_NA (PERF_MEM_S(OP, NA) |\ diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index b0c45d923f0f..f32c623abef6 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -923,7 +923,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map, * bpf_perf_event_output */ struct bpf_trace_sample_data { - struct perf_sample_data sds[3]; + perf_sample_data_t sds[3]; }; static DEFINE_PER_CPU(struct bpf_trace_sample_data, bpf_trace_sds);