From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [RFC perf,bpf 1/5] perf, bpf: Introduce PERF_RECORD_BPF_EVENT Date: Thu, 8 Nov 2018 16:00:28 +0100 Message-ID: <20181108150028.GU9761@hirez.programming.kicks-ass.net> References: <20181106205246.567448-1-songliubraving@fb.com> <20181106205246.567448-2-songliubraving@fb.com> <20181107084057.GG9781@hirez.programming.kicks-ass.net> <31067290-4B66-4AA1-8027-607397BC0264@fb.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Netdev , lkml , Kernel Team , "ast@kernel.org" , "daniel@iogearbox.net" , "acme@kernel.org" To: Song Liu Return-path: Content-Disposition: inline In-Reply-To: <31067290-4B66-4AA1-8027-607397BC0264@fb.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Nov 07, 2018 at 06:25:04PM +0000, Song Liu wrote: > > > > On Nov 7, 2018, at 12:40 AM, Peter Zijlstra wrote: > > > > On Tue, Nov 06, 2018 at 12:52:42PM -0800, Song Liu wrote: > >> For better performance analysis of BPF programs, this patch introduces > >> PERF_RECORD_BPF_EVENT, a new perf_event_type that exposes BPF program > >> load/unload information to user space. > >> > >> /* > >> * Record different types of bpf events: > >> * enum perf_bpf_event_type { > >> * PERF_BPF_EVENT_UNKNOWN = 0, > >> * PERF_BPF_EVENT_PROG_LOAD = 1, > >> * PERF_BPF_EVENT_PROG_UNLOAD = 2, > >> * }; > >> * > >> * struct { > >> * struct perf_event_header header; > >> * u16 type; > >> * u16 flags; > >> * u32 id; // prog_id or map_id > >> * }; > >> */ > >> PERF_RECORD_BPF_EVENT = 17, > >> > >> PERF_RECORD_BPF_EVENT contains minimal information about the BPF program. > >> Perf utility (or other user space tools) should listen to this event and > >> fetch more details about the event via BPF syscalls > >> (BPF_PROG_GET_FD_BY_ID, BPF_OBJ_GET_INFO_BY_FD, etc.). > > > > Why !? You're failing to explain why it cannot provide the full > > information there. > > Aha, I missed this part. I will add the following to next version. Please > let me know if anything is not clear. > > This design decision is picked for the following reasons. First, BPF > programs could be loaded-and-jited and/or unloaded before/during/after > perf-record run. Once a BPF programs is unloaded, it is impossible to > recover details of the program. It is impossible to provide the > information through a simple key (like the build ID). Second, BPF prog > annotation is under fast developments. Multiple informations will be > added to bpf_prog_info in the next few releases. Including all the > information of a BPF program in the perf ring buffer requires frequent > changes to the perf ABI, and thus makes it very difficult to manage > compatibility of perf utility. So I don't agree with that reasoning. If you want symbol information you'll just have to commit to some form of ABI. That bpf_prog_info is an ABI too. And relying on userspace to synchronously consume perf output to directly call into the kernel again to get more info (through another ABI) is a pretty terrible design. So please try harder. NAK on this.