Messages in this thread | | | Date | Thu, 10 Jan 2019 12:25:13 -0500 (EST) | From | Mathieu Desnoyers <> | Subject | Re: Possible use of RCU while in extended QS: idle vs RCU read-side in interrupt vs rcu_eqs_exit |
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----- On Jan 10, 2019, at 9:11 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
> ----- On Jan 10, 2019, at 8:44 AM, Mathieu Desnoyers > mathieu.desnoyers@efficios.com wrote: > >> ----- On Jan 10, 2019, at 8:08 AM, rostedt rostedt@goodmis.org wrote: >> >>> On Wed, 9 Jan 2019 20:38:51 -0500 (EST) >>> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote: >>> >>>> Hi Paul, >>>> >>>> I've had a user report that trace_sched_waking() appears to be >>>> invoked while !rcu_is_watching() in some situation, so I started >>>> digging into the scheduler idle code. >>> >>> I'm wondering if this isn't a bug. Do you have the backtrace for where >>> trace_sched_waking() was called without rcu watching? >> >> I strongly suspect a bug as well. I'm awaiting a reproducer from the >> user whom reported this issue so I can add a WARN_ON_ONCE(!rcu_is_watching()) >> in the scheduler code near trace_sched_waking() and gather a backtrace. >> >> It still has to be confirmed, but I suspect this have been triggered >> within a HyperV guest. It may therefore be related to a virtualized environment. >> >> I'll try to ask more specifically on which environment this was encountered. > > So it ends up it happens directly on hardware on a Linux laptop. Here is > the stacktrace: > > vmlinux!try_to_wake_up > vmlinux!default_wake_function > vmlinux!pollwake > vmlinux!__wake_up_common > vmlinux!__wake_up_common_lock > vmlinux!__wake_up > vmlinux!perf_event_wakeup > vmlinux!perf_pending_event > vmlinux!irq_work_run_list > vmlinux!irq_work_run > vmlinux!smp_irq_work_iterrupt > vmlinux!irq_work_interrupt > vmlinux!finish_task_switch > vmlinux!__schedule > vmlinux!schedule_idle > vmlinux!do_idle > vmlinux!cpu_startup_entry > vmlinux!start_secondary > vmlinux!secondary_startup_64 > > Does it raise any red flag ?
Based on this backtrace, I think I start to get a better understanding of the situation.
The initial problem reported to me was that ftrace was showing some sched_waking events in its trace that were missed by perf.
I presumed this was because of the !rcu_is_watching() check, but I think I was wrong.
This backtrace seems to show that perf is itself triggering a sched_waking() event. It there is probably a check that discards nested events within perf, which would discard this from perf traces, but ftrace (and lttng) would trace it just fine.
Thoughts ?
Thanks,
Mathieu
> > Thanks, > > Mathieu > >> >> Thanks, >> >> Mathieu >> >>> >>> -- Steve >>> >>>> >>>> It appears that interrupts are re-enabled before rcu_eqs_exit() is >>>> invoked when exiting idle code from the scheduler. >>>> >>>> I wonder what happens if an interrupt handler (including scheduler code) >>>> happens to issue a RCU read-side critical section before rcu_eqs_exit() >>>> is called ? Is there some code on interrupt entry that ensures rcu eqs >>>> state is exited in such scenario ? >>>> >>>> Thanks, >>>> >>>> Mathieu >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. >> http://www.efficios.com > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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