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=-5.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 4833EC43460 for ; Fri, 16 Apr 2021 09:27:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1872D61166 for ; Fri, 16 Apr 2021 09:27:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237457AbhDPJ1t (ORCPT ); Fri, 16 Apr 2021 05:27:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60782 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231666AbhDPJ1r (ORCPT ); Fri, 16 Apr 2021 05:27:47 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 28BC6C061574 for ; Fri, 16 Apr 2021 02:27:23 -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-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=HPc6wzpM0fRvoMm2lRghdfZ9iKMJxYwnHLDM6jmTY5Y=; b=GDTZlHwLb/nSS3o8n4T46sXmHr B0QkYBvpxyMVglHu0XAuU8Uoea3LJTV7EM1KE2rOVYoGojgP0gX87RLO0/1W3zteJVJexcOit7wBh MeLYwj9Ir1YpYdHpbfaqQxsbs94cFBo3y27FuYj6QAN9s32ZB+WJ9Emf33DDq0ArlrhQT6ZVT68Iw zPfjqoF82EJrq4sZYj4CPIdIb10zjQXz0mWAjGr77le9MHKoy+K4Q/fi9O+uYNtRw0c4D0jeJf+Wv Qo7moPKS4P2ApUpu3SueKgs3XQ3ng8CLcTBJCpsp2Gm72qiYI2s52RFffBA9Od5uMdyDVWpSFKY98 EHR9Ko8Q==; 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 1lXKkL-009lQi-Qd; Fri, 16 Apr 2021 09:26:44 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 1DECE30015A; Fri, 16 Apr 2021 11:26:39 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id A98D5203711CC; Fri, 16 Apr 2021 11:26:39 +0200 (CEST) Date: Fri, 16 Apr 2021 11:26:39 +0200 From: Peter Zijlstra To: Namhyung Kim Cc: Ingo Molnar , Arnaldo Carvalho de Melo , Jiri Olsa , Mark Rutland , Alexander Shishkin , LKML , Stephane Eranian , Andi Kleen , Ian Rogers , Song Liu , Tejun Heo , kernel test robot , Thomas Gleixner Subject: Re: [PATCH v3 1/2] perf/core: Share an event with multiple cgroups Message-ID: References: <20210413155337.644993-1-namhyung@kernel.org> <20210413155337.644993-2-namhyung@kernel.org> 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, Apr 16, 2021 at 08:48:12AM +0900, Namhyung Kim wrote: > On Thu, Apr 15, 2021 at 11:51 PM Peter Zijlstra wrote: > > On Tue, Apr 13, 2021 at 08:53:36AM -0700, Namhyung Kim wrote: > > > cgroup event counting (i.e. perf stat). > > > > > > * PERF_EVENT_IOC_ATTACH_CGROUP - it takes a buffer consists of a > > > 64-bit array to attach given cgroups. The first element is a > > > number of cgroups in the buffer, and the rest is a list of cgroup > > > ids to add a cgroup info to the given event. > > > > WTH is a cgroup-id? The syscall takes a fd to the path, why have two > > different ways? > > As you know, we already use cgroup-id for sampling. Yeah we > can do it with the fd but one of the point in this patch is to reduce > the number of file descriptors. :) Well, I found those patches again after I wrote that. But I'm still not sure what a cgroup-id is from userspace. How does userspace get one given a cgroup? (I actually mounted cgroupfs in order to see if there's some new 'id' file to read, there is not) Does having the cgroup-id ensure the cgroup exists? Can the cgroup-id get re-used? I really don't konw what the thing is. I don't use cgroups, like ever, except when I'm forced to due to some regression or bugreport. > Also, having cgroup-id is good to match with the result (from read) > as it contains the cgroup information. What? > > > * PERF_EVENT_IOC_READ_CGROUP - it takes a buffer consists of a 64-bit > > > array to get the event counter values. The first element is size > > > of the array in byte, and the second element is a cgroup id to > > > read. The rest is to save the counter value and timings. > > > > :-( > > > > So basically you're doing a whole seconds cgroup interface, one that > > violates the one counter per file premise and lives off of ioctl()s. > > Right, but I'm not sure that we really want a separate event for each > cgroup if underlying hardware events are all the same. Sure, I see where you're coming from; I just don't much like where it got you :-) > > *IF* we're going to do something like this, I feel we should explore the > > whole vector-per-fd concept before proceeding. Can we make it less yuck > > (less special ioctl() and more regular file ops. Can we apply the > > concept to more things? > > Ideally it'd do without keeping file descriptors open. Maybe we can make > the vector accept various types like vector-per-cgroup_id or so. So I think we've had proposals for being able to close fds in the past; while preserving groups etc. We've always pushed back on that because of the resource limit issue. By having each counter be a filedesc we get a natural limit on the amount of resources you can consume. And in that respect, having to use 400k fds is things working as designed. Anyway, there might be a way around this.. > > The second patch extends the ioctl() to be more read() like, instead of > > doing the sane things and extending read() by adding PERF_FORMAT_VECTOR > > or whatever. In fact, this whole second ioctl() doesn't make sense to > > have if we do indeed want to do vector-per-fd. > > One of the upside of the ioctl() is that we can pass cgroup-id to read. > Probably we can keep the index in the vector and set the file offset > with it. Or else just read the whole vector, and then it has a cgroup-id > in the output like PERF_FORMAT_CGROUP? > > > > > Also, I suppose you can already fake this, by having a > > SW_CGROUP_SWITCHES (sorry, I though I picked those up, done now) event > > Thanks! > > > with PERF_SAMPLE_READ|PERF_SAMPLE_CGROUP and PERF_FORMAT_GROUP in a > > group with a bunch of events. Then the buffer will fill with the values > > you use here. > > Right, I'll do an experiment with it. > > > > > Yes, I suppose it has higher overhead, but you get the data you want > > without having to do terrible things like this. > > That's true. And we don't need many things in the perf record like > synthesizing task/mmap info. Also there's a risk we can miss some > samples for some reason. > > Another concern is that it'd add huge slow down in the perf event > open as it creates a mixed sw/hw group. The synchronized_rcu in > the move_cgroup path caused significant problems in my > environment as it adds up in proportion to the number of cpus. Since when is perf_event_open() a performance concern? That thing is slow in all possible ways.