linux-kernel - [PATCH 3.12 85/98] tracing: Have preempt(irqs)off trace preempt disabled functions

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Message-Id: <91d4aae2b4ac1abc1009309d19604b9c88d60915.1460380917.git.jslaby@suse.cz>
Date:	Mon, 11 Apr 2016 15:23:27 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 85/98] tracing: Have preempt(irqs)off trace preempt disabled functions

From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit cb86e05390debcc084cfdb0a71ed4c5dbbec517d upstream.

Joel Fernandes reported that the function tracing of preempt disabled
sections was not being reported when running either the preemptirqsoff or
preemptoff tracers. This was due to the fact that the function tracer
callback for those tracers checked if irqs were disabled before tracing. But
this fails when we want to trace preempt off locations as well.

Joel explained that he wanted to see funcitons where interrupts are enabled
but preemption was disabled. The expected output he wanted:

   <...>-2265    1d.h1 3419us : preempt_count_sub <-irq_exit
   <...>-2265    1d..1 3419us : __do_softirq <-irq_exit
   <...>-2265    1d..1 3419us : msecs_to_jiffies <-__do_softirq
   <...>-2265    1d..1 3420us : irqtime_account_irq <-__do_softirq
   <...>-2265    1d..1 3420us : __local_bh_disable_ip <-__do_softirq
   <...>-2265    1..s1 3421us : run_timer_softirq <-__do_softirq
   <...>-2265    1..s1 3421us : hrtimer_run_pending <-run_timer_softirq
   <...>-2265    1..s1 3421us : _raw_spin_lock_irq <-run_timer_softirq
   <...>-2265    1d.s1 3422us : preempt_count_add <-_raw_spin_lock_irq
   <...>-2265    1d.s2 3422us : _raw_spin_unlock_irq <-run_timer_softirq
   <...>-2265    1..s2 3422us : preempt_count_sub <-_raw_spin_unlock_irq
   <...>-2265    1..s1 3423us : rcu_bh_qs <-__do_softirq
   <...>-2265    1d.s1 3423us : irqtime_account_irq <-__do_softirq
   <...>-2265    1d.s1 3423us : __local_bh_enable <-__do_softirq

There's a comment saying that the irq disabled check is because there's a
possible race that tracing_cpu may be set when the function is executed. But
I don't remember that race. For now, I added a check for preemption being
enabled too to not record the function, as there would be no race if that
was the case. I need to re-investigate this, as I'm now thinking that the
tracing_cpu will always be correct. But no harm in keeping the check for
now, except for the slight performance hit.

Link: http://lkml.kernel.org/r/1457770386-88717-1-git-send-email-agnel.joel@gmail.com

Fixes: 5e6d2b9cfa3a "tracing: Use one prologue for the preempt irqs off tracer function tracers"
Reported-by: Joel Fernandes <agnel.joel@...il.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 kernel/trace/trace_irqsoff.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index 2aefbee93a6d..56e083e26ca9 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -118,8 +118,12 @@ static int func_prolog_dec(struct trace_array *tr,
 		return 0;

 	local_save_flags(*flags);
-	/* slight chance to get a false positive on tracing_cpu */
-	if (!irqs_disabled_flags(*flags))
+	/*
+	 * Slight chance to get a false positive on tracing_cpu,
+	 * although I'm starting to think there isn't a chance.
+	 * Leave this for now just to be paranoid.
+	 */
+	if (!irqs_disabled_flags(*flags) && !preempt_count())
 		return 0;

 	*data = per_cpu_ptr(tr->trace_buffer.data, cpu);
-- 
2.8.1