From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966337AbcHEBnw (ORCPT ); Thu, 4 Aug 2016 21:43:52 -0400 Received: from mail-pf0-f196.google.com ([209.85.192.196]:33798 "EHLO mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965272AbcHEBnt (ORCPT ); Thu, 4 Aug 2016 21:43:49 -0400 Date: Thu, 4 Aug 2016 18:43:43 -0700 From: Alexei Starovoitov To: Peter Zijlstra Cc: Brendan Gregg , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , linux-kernel@vger.kernel.org, Alexei Starovoitov , Wang Nan Subject: Re: [PATCH v2 1/3] perf/core: Add a tracepoint for perf sampling Message-ID: <20160805014341.GB52225@ast-mbp.thefacebook.com> References: <1470192469-11910-1-git-send-email-bgregg@netflix.com> <1470192469-11910-2-git-send-email-bgregg@netflix.com> <20160803094805.GD6879@twins.programming.kicks-ass.net> <20160804142853.GO6862@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160804142853.GO6862@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 04, 2016 at 04:28:53PM +0200, Peter Zijlstra wrote: > On Wed, Aug 03, 2016 at 11:57:05AM -0700, Brendan Gregg wrote: > > > As for pmu tracepoints: if I were to instrument it (although I wasn't > > planning to), I'd put a tracepoint in perf_event_overflow() called > > "perf:perf_overflow", with the same arguments. That could then be used > > for all PMU overflow events, without needing to add specific > > tracepoints. > > Could we not teach BPF to replace event->overflow_handler and inject > itself there? > > We don't currently have nice interfaces for doing that, but it should be > possible to do I think. We already have the indirect function call, so > injecting ourself there has 0 overhead. you're right. All makes sense. I guess I was too lazy to look into how to do it properly. Adding a tracepoint looked like quick and easy way to achieve the same. As far as api goes probably existing IOC_SET_BPF ioctl will do too. Currently overflow_handler is set at event alloc time. If we start changing it on the fly with atomic xchg(), afaik things shouldn't break, since each overflow_handler is run to completion and doesn't change global state, right?