Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1757047AbcCaOor (ORCPT <rfc822;w@1wt.eu>);
	Thu, 31 Mar 2016 10:44:47 -0400
Received: from mail.kernel.org ([198.145.29.136]:59498 "EHLO mail.kernel.org"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753785AbcCaOoq (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Thu, 31 Mar 2016 10:44:46 -0400
Date: Thu, 31 Mar 2016 11:44:39 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: kan.liang@intel.com, ak@linux.intel.com, eranian@google.com,
        vincent.weaver@maine.edu, tglx@linutronix.de, mingo@kernel.org,
        acme@redhat.com, jolsa@redhat.com, alexander.shishkin@linux.intel.com,
        ying.huang@linux.intel.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2 1/1] perf/core: don't find side-band event from all
 pmus
Message-ID: <20160331144439.GB27708@kernel.org>
References: <1458757477-3781-1-git-send-email-kan.liang@intel.com>
 <20160329120609.GG3408@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160329120609.GG3408@twins.programming.kicks-ass.net>
X-Url: http://acmel.wordpress.com
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 3115
Lines: 86

Em Tue, Mar 29, 2016 at 02:06:09PM +0200, Peter Zijlstra escreveu:
> On Wed, Mar 23, 2016 at 11:24:37AM -0700, kan.liang@intel.com wrote:
> > The V2 patch is mainly based on Peter's suggestion. But I didn't rename
> > perf_event_aux to perf_event_sb. Because it looks there are many aux things
> > in the codes, e.g. AUX area in ring buffer. I'm not sure if we need to change
> > all aux to sb. We may do the rename later in separate patch.
> 
> Right.. no problem doing that in a separate patch.
> 
> > +static void perf_event_sb_mask(unsigned int sb_mask,
> > +			       perf_event_aux_output_cb output,
> > +			       void *data)
> > +{
> > +	int sb;
> > +
> > +	for (sb = 0; sb < sb_nr; sb++) {
> > +		if (!(sb_mask & (1 << sb)))
> > +			continue;
> > +		perf_event_sb_iterate(sb, output, data);
> > +	}
> > +}
> 
> > @@ -5852,7 +5910,8 @@ static void perf_event_task(struct task_struct *task,
> >  
> >  	perf_event_aux(perf_event_task_output,
> >  		       &task_event,
> > -		       task_ctx);
> > +		       task_ctx,
> > +		       (1 << sb_task) | (1 << sb_mmap) | (1 << sb_comm));
> >  }
> 
> So one side-effect of this change is that the above event can be
> delivered 3 times if you're 'lucky'.
> 
> Acme; does userspace care?

So, when processing a PERF_RECORD_FORK there is some sanity checks in
machine__process_fork_event() (tools/perf/util/machine.c), and one that
is affected by copies is:

        struct thread *thread = machine__find_thread(machine,
                                                     event->fork.pid,
                                                     event->fork.tid);
<SNIP>
        /* if a thread currently exists for the thread id remove it */
        if (thread != NULL) {
                machine__remove_thread(machine, thread);
                thread__put(thread);
        }

        thread = machine__findnew_thread(machine, event->fork.pid,
                                         event->fork.tid);
<SNIP>

So we conceivably may end up with multiple 'struct thread' pointing to
the same kernel thread, being held as references somewhere (in a
hist_entry, for instance, if a PERF_RECORD_SAMPLE happens mid sentence).

It probably will cope, but can't we just emit one single record?

PERF_RECORD_EXIT are ok:

int machine__process_exit_event(struct machine *machine, union perf_event *event,
                                struct perf_sample *sample __maybe_unused)
{
        struct thread *thread = machine__find_thread(machine,
                                                     event->fork.pid,
                                                     event->fork.tid);

        if (dump_trace)
                perf_event__fprintf_task(event, stdout);

        if (thread != NULL) {
                thread__exited(thread);
                thread__put(thread);
        }

        return 0;
}

PERF_RECORD_COMM will unecessarily add a new COMM to its list, leading tooling
to think that there was a prctl(PR_SET_NAME). This could conceivably be annoyed
away by checking if the "new" COMM is the current one, again, can't the kernel
emit just one record?

- Arnaldo