Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753159AbdF3VYu (ORCPT ); Fri, 30 Jun 2017 17:24:50 -0400 Received: from mail-oi0-f52.google.com ([209.85.218.52]:35877 "EHLO mail-oi0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753142AbdF3VYt (ORCPT ); Fri, 30 Jun 2017 17:24:49 -0400 MIME-Version: 1.0 In-Reply-To: <20170630170740.7a0eeaf5@gandalf.local.home> References: <20170412053851.26286-1-joelaf@google.com> <20170630085148.4eb252de@gandalf.local.home> <20170630170740.7a0eeaf5@gandalf.local.home> From: Joel Fernandes Date: Fri, 30 Jun 2017 14:24:47 -0700 Message-ID: Subject: Re: [RFC] tracing: Add support for critical section event tracing To: Steven Rostedt Cc: LKML Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1743 Lines: 47 On Fri, Jun 30, 2017 at 2:07 PM, Steven Rostedt wrote: > On Fri, 30 Jun 2017 14:03:14 -0700 > Joel Fernandes wrote: > >> Hi Steven, >> >> Thanks a lot for the comments, I agree with all of them and had a >> comment about one of them: >> >> On Fri, Jun 30, 2017 at 5:51 AM, Steven Rostedt wrote: >> [..] >> > Are you not worried about recursion here? There's no protection. >> > Wouldn't it be better to have: >> > >> > if (!this_cpu_read(tracing_events_cpu)) >> > return; >> > >> > trace_critical_end(ip, parent_ip); >> > >> > this_cpu_write(tracing_events_cpu, 0); >> > >> > ? >> > >> >> I tried to go over some scenarios and I think it shouldn't be a >> problem because we start the critical event only when either >> interrupts are turned off while preemption is turned on, or preempt is >> turned off while interrupts are turned on, and the fact that we call >> the tracer while still in the critical section. Let me know if you had >> a scenario in mind that can cause problems with this. > > Then may I ask what is tracing_events_cpu actually protecting? Yes actually I think its not needed considering the above. When I was developing the code I wanted to do something similar start_critical_timing which has a tracing_cpu per-cpu variable, and at the time wasn't sure if it was needed so I just added it in anyway. Since the case of this patch is more specific (both preempt and irq turned back on to mark end of critical section), it may not be needed and I can drop it. I will think some more about it as well let you know if I find a scenario that says otherwise. Thanks so much for making the patch better!! Regards, Joel