linux-kernel - [PATCH -next] tracepoint: ipi: don't trace IPI on offline CPUs

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Date:	Wed,  9 Mar 2016 12:22:22 +0000
From:	Sudeep Holla <sudeep.holla@....com>
To:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Cc:	Sudeep Holla <sudeep.holla@....com>, Ingo Molnar <mingo@...hat.com>
Subject: [PATCH -next] tracepoint: ipi: don't trace IPI on offline CPUs

Tracepoints use RCU for protection and they must not be called on
offline CPUS. So make this tracepoint conditional. Otherwise it can
produce the following warning:

===============================
[ INFO: suspicious RCU usage. ]
4.5.0-rc7-next-20160308 #44 Not tainted
-------------------------------
include/trace/events/ipi.h:35 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

RCU used illegally from offline CPU!
rcu_scheduler_active = 1, debug_locks = 1
no locks held by swapper/3/0.

stack backtrace:
CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.5.0-rc7-next-20160308 #44
Hardware name: ARM Juno development board (r2) (DT)
Call trace:
 dump_backtrace+0x0/0x218
 show_stack+0x24/0x30
 dump_stack+0xb4/0xf0
 lockdep_rcu_suspicious+0xd0/0x118
 smp_cross_call+0x1a8/0x1b0
 arch_send_call_function_single_ipi+0x3c/0x48
 generic_exec_single+0xc0/0x178
 smp_call_function_single+0xec/0x1b8
 cpuhp_report_idle_dead+0x70/0x88
 cpu_startup_entry+0x358/0x3c0
 secondary_start_kernel+0x190/0x1e8

Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>
Signed-off-by: Sudeep Holla <sudeep.holla@....com>
---
 include/trace/events/ipi.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Hi Steven,

I observed that in "include/linux/tracepoint.h", we have
#define __DO_TRACE(tp, proto, args, cond, prercu, postrcu)
...
                 if (!cpu_online(raw_smp_processor_id()))
                         return;

                 if (!(cond))
                         return;
...

where !cond check seems reduntant if it's cpu_online check.
So, does this patch handle the warning correctly or is there any better
way ? I did see few traces with same condition, just thought of checking
with you.

Regards,
Sudeep

diff --git a/include/trace/events/ipi.h b/include/trace/events/ipi.h
index 834a7362a610..d8ca2f2e24ac 100644
--- a/include/trace/events/ipi.h
+++ b/include/trace/events/ipi.h
@@ -15,12 +15,14 @@
  * It is necessary for @reason to be a static string declared with
  * __tracepoint_string.
  */
-TRACE_EVENT(ipi_raise,
+TRACE_EVENT_CONDITION(ipi_raise,

 	TP_PROTO(const struct cpumask *mask, const char *reason),

 	TP_ARGS(mask, reason),

+	TP_CONDITION(cpu_online(raw_smp_processor_id())),
+
 	TP_STRUCT__entry(
 		__bitmask(target_cpus, nr_cpumask_bits)
 		__field(const char *, reason)
--
1.9.1