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 D487FC433F5 for ; Wed, 27 Apr 2022 10:55:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231177AbiD0K6Y (ORCPT ); Wed, 27 Apr 2022 06:58:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59888 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230422AbiD0K6V (ORCPT ); Wed, 27 Apr 2022 06:58:21 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 4A19C3C19A3 for ; Wed, 27 Apr 2022 03:35:04 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E920C143D; Wed, 27 Apr 2022 03:35:03 -0700 (PDT) Received: from wubuntu (unknown [10.57.77.199]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 578373F774; Wed, 27 Apr 2022 03:35:00 -0700 (PDT) Date: Wed, 27 Apr 2022 11:34:58 +0100 From: Qais Yousef To: Andrii Nakryiko Cc: Peter Zijlstra , Alexei Starovoitov , 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: <20220427103458.ecnqtaj3af63625h@wubuntu> References: <20220120162520.570782-1-valentin.schneider@arm.com> <93a20759600c05b6d9e4359a1517c88e06b44834.camel@fb.com> <20220422110903.GW2731@worktop.programming.kicks-ass.net> <056e9bb0d0e3fc20572d42db7386face1d0665d6.camel@fb.com> <20220426140959.op6u5m7id57aq7yc@wubuntu> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/26/22 08:54, Andrii Nakryiko wrote: > On Tue, Apr 26, 2022 at 7:10 AM Qais Yousef wrote: > > > > On 04/26/22 14:28, Peter Zijlstra wrote: > > > 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? > > > > If libbpf doesn't fail, then yeah it's a big problem. I wonder how users of > > kprobe who I suppose are more prone to this kind of problems have been coping. > > See my reply to Peter. libbpf can't know user's intent to fail this > automatically, in general. In some cases when it can it does > accommodate this automatically. In other cases it provides instruments > for user to handle this (bpf_core_field_size(), > BPF_CORE_READ_BITFIELD(), etc). My naiive thinking is that the function signature has changed (there's 1 extra arg not just a subtle swap of args of the same type) - so I thought that can be detected. But maybe it is harder said than done. I am trying to remember as I've used this before; I think you get the arg list as part of ctx when you attach to a function? I wonder if it'd be hard to provide a macro for the user to provide the signature of the function they expect; this macro can try then to verify/assert the number, type and order is the same. Not bullet proof and requires opt-in, but could be useful? // dummy pseudo-code BPF_CORE_ASSERT_SIG(sched_switch, NR_ARGS, ARG0, ARG1, ...) if (ctx->nr_args != NR_ARGS) assert() if (type_of(ctx->args[0]) != type_of(ARG0)) assert() ... I'm not sure if you have any info about the type though.. > But in the end no one eliminated the need for testing your application > for correctness. Tracing programs do break on kernel changes and BPF > users do adapt to them. Sometimes adapting is easy (like state -> > __state transition), sometimes it's much more involved (like this > argument order change). It's not just an arg re-order, it's a new argument inserted in the middle. But fair enough :-) Cheers -- Qais Yousef