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Date: Wed, 10 Dec 2014 17:44:28 -0500 From: Steven Rostedt <rostedt@...dmis.org> To: Peter Zijlstra <peterz@...radead.org> Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de> Subject: [PATCH] tracing/sched: Check preempt_count() for current when reading task->state When recording the state of a task for the sched_switch tracepoint a check of task_preempt_count() is performed to see if PREEMPT_ACTIVE is set. This is because, technically, a task being preempted is really in the TASK_RUNNING state, and that is what should be recorded when tracing a sched_switch, even if the task put itself into another state (it hasn't scheduled out in that state yet). But with the change to use per_cpu preempt counts, the task_thread_info(p)->preempt_count is no longer used, and instead task_preempt_count(p) is used. The problem is that this does not use the current preempt count but a stale one from a previous sched_switch. The task_preempt_count(p) uses saved_preempt_count and not preempt_count(). But for tracing sched_switch, if p is current, we really want preempt_count(). I hit this bug when I was tracing sleep and the call from do_nanosleep() scheduled out in the "RUNNING" state. sleep-4290 [000] 537272.259992: sched_switch: sleep:4290 [120] R ==> swapper/0:0 [120] sleep-4290 [000] 537272.260015: kernel_stack: <stack trace> => __schedule (ffffffff8150864a) => schedule (ffffffff815089f8) => do_nanosleep (ffffffff8150b76c) => hrtimer_nanosleep (ffffffff8108d66b) => SyS_nanosleep (ffffffff8108d750) => return_to_handler (ffffffff8150e8e5) => tracesys_phase2 (ffffffff8150c844) After a bit of hair pulling, I found that the state was really TASK_INTERRUPTIBLE, but the saved_preempt_count had an old PREEMPT_ACTIVE set and caused the sched_switch tracepoint to show it as RUNNING. Cc: stable@...r.kernel.org # 3.13+ Fixes: 01028747559a "sched: Create more preempt_count accessors" Signed-off-by: Steven Rostedt <rostedt@...dmis.org> --- include/trace/events/sched.h | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h index 0a68d5ae584e..13fbadcc172b 100644 --- a/include/trace/events/sched.h +++ b/include/trace/events/sched.h @@ -97,10 +97,14 @@ static inline long __trace_sched_switch_state(struct task_struct *p) long state = p->state; #ifdef CONFIG_PREEMPT + unsigned long pc; + + pc = (p == current) ? preempt_count() : task_preempt_count(p); + /* * For all intents and purposes a preempted task is a running task. */ - if (task_preempt_count(p) & PREEMPT_ACTIVE) + if (pc & PREEMPT_ACTIVE) state = TASK_RUNNING | TASK_STATE_MAX; #endif -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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