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 634B9C433F5 for ; Tue, 26 Apr 2022 12:28:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348926AbiDZMbo (ORCPT ); Tue, 26 Apr 2022 08:31:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39310 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232492AbiDZMbm (ORCPT ); Tue, 26 Apr 2022 08:31:42 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DFD1E65400 for ; Tue, 26 Apr 2022 05:28:34 -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=ew8SaulpXXHWoXL4r2sDpyXdeIciKedNMV91X3hIzrM=; b=Q+SIF2W1zVEsX1sPyqeOq870bU 1Oa95N09daMlfQCJuWYIc50tOUoU/qRDdUEjo7dXOkRd5vNK3+XUhaCyQRVFTObbyyBopAHf48zFL 7LeFM03w9+D9hjxJpaRTi81NZOhjnbiDUKV2hxLOZXnykF1jz3703yTsrv2nSzd8+vrCWrsCVR/bK oMtVUkxN3S69YReuVqH0Euc4/dxu8+S4xb9TI0ghLlidAS2bM3kxubJx3ARU8qi5v4TqmKHYRpXTd GU1jBq7pshNnUf4Xql8OH+bu7l3xYLK5LZ3nan9QZ9zxaLlO92FDX7HZdxsi0+Q248CebJmyVZ8Nv iIbY9nFQ==; 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 #2 (Red Hat Linux)) id 1njKIY-009csb-U0; Tue, 26 Apr 2022 12:28:07 +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 (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 6F1BD30031D; Tue, 26 Apr 2022 14:28:04 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 469722029F9FF; Tue, 26 Apr 2022 14:28:04 +0200 (CEST) Date: Tue, 26 Apr 2022 14:28:04 +0200 From: Peter Zijlstra To: Alexei Starovoitov Cc: Delyan Kratunov , Namhyung Kim , Arnaldo Carvalho de Melo , "bigeasy@linutronix.de" , "dietmar.eggemann@arm.com" , "keescook@chromium.org" , "x86@kernel.org" , "andrii@kernel.org" , "u.kleine-koenig@pengutronix.de" , "vincent.guittot@linaro.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "linux-kernel@vger.kernel.org" , "rdunlap@infradead.org" , "rostedt@goodmis.org" , "Kenta.Tada@sony.com" , "tglx@linutronix.de" , "bristot@redhat.com" , "ebiederm@xmission.com" , "ast@kernel.org" , "legion@kernel.org" , "adharmap@quicinc.com" , "valentin.schneider@arm.com" , "ed.tsai@mediatek.com" , "juri.lelli@redhat.com" Subject: Re: [PATCH] sched/tracing: append prev_state to tp args instead Message-ID: References: <20220120162520.570782-1-valentin.schneider@arm.com> <93a20759600c05b6d9e4359a1517c88e06b44834.camel@fb.com> <20220422110903.GW2731@worktop.programming.kicks-ass.net> <056e9bb0d0e3fc20572d42db7386face1d0665d6.camel@fb.com> 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 22, 2022 at 11:30:12AM -0700, Alexei Starovoitov wrote: > On Fri, Apr 22, 2022 at 10:22 AM Delyan Kratunov wrote: > > > > On Fri, 2022-04-22 at 13:09 +0200, Peter Zijlstra wrote: > > > And on the other hand; those users need to be fixed anyway, right? > > > Accessing prev->__state is equally broken. > > > > The users that access prev->__state would most likely have to be fixed, for sure. > > > > However, not all users access prev->__state. `offcputime` for example just takes a > > stack trace and associates it with the switched out task. This kind of user > > would continue working with the proposed patch. > > > > > If bpf wants to ride on them, it needs to suffer the pain of doing so. > > > > Sure, I'm just advocating for a fairly trivial patch to avoid some of the suffering, > > hopefully without being a burden to development. If that's not the case, then it's a > > clear no-go. > > > Namhyung just sent this patch set: > https://patchwork.kernel.org/project/netdevbpf/patch/20220422053401.208207-3-namhyung@kernel.org/ That has: + * recently task_struct->state renamed to __state so it made an incompatible + * change. git tells me: 2f064a59a11f ("sched: Change task_struct::state") is almost a year old by now. That don't qualify as recently in my book. That says that 'old kernels used to call this...'. > to add off-cpu profiling to perf. > It also hooks into sched_switch tracepoint. > Notice it deals with state->__state rename just fine. So I don't speak BPF much; it always takes me more time to make bpf work than to just hack up the kernel, which makes it hard to get motivated. However, it was not just a rename, state changed type too, which is why I did the rename, to make sure all users would get a compile fail and could adjust. If you're silently making it work by frobbing the name, you loose that. Specifically, task_struct::state used to be 'volatile long', while task_struct::__state is 'unsigned int'. As such, any user must now be very careful to use READ_ONCE(). I don't see that happening with just frobbing the name. Additinoally, by shrinking the field, I suppose BE systems get to keep the pieces? > But it will have a hard time without this patch > until we add all the extra CO-RE features to detect > and automatically adjust bpf progs when tracepoint > arguments order changed. Could be me, but silently making it work sounds like fail :/ There's a reason code changes, users need to adapt, not silently pretend stuff is as before. How will you know you need to fix your tool? > We will do it eventually, of course. > There will be additional work in llvm, libbpf, kernel, etc. > But for now I think it would be good to land Delyan's patch > to avoid unnecessary pain to all the users. > > Peter, do you mind? I suppose I can help out this time, but I really don't want to set a precedent for these things. Broken is broken. The down-side for me is that the argument order no longer makes any sense.