----- On Jun 22, 2022, at 4:28 PM, Mathieu Desnoyers mathieu.desnoy...@efficios.com wrote:
> ----- On Jun 22, 2022, at 9:19 AM, Mathieu Desnoyers > mathieu.desnoy...@efficios.com wrote: > >> ----- On Jun 22, 2022, at 3:45 AM, Minlan Wang wangmin...@szsandstone.com >> wrote: >> >> [...] >> >> Hi Minlan, >> >>> -- >>> 1.8.3.1 >>> >>> >>> And the lttng session is configured to trace these events: >>> kernel: syscall futex >> >> On the kernel side, in addition to the syscall futex, I would really like to >> see >> what >> happens in the scheduler, mainly the wait/wakeup tracepoints. This can be >> added >> by using: >> >> lttng enable-event -k 'sched_*' >> >> This should help us confirm whether we indeed have a situation where queued >> wake >> ups >> happen to wake up a wait happening only later, which is unexpected by the >> current liburcu >> userspace code. >> >> [...] >> >>> --- >>> >>> The babletrace output of this session is pretty big, 6 MB in size, i put it >>> in >>> the attachment trace_0622.tar.bz2. >>> Let my know if your mailbox can't handle such big attachment. >> >> It would be even better if you can share the binary trace, because then it's >> easy to >> load it in trace compass, cut away time ranges that don't matter, and lots of >> other >> useful stuff. > > I just found the relevant snippet of documentation in futex(5): > > FUTEX_WAIT > Returns 0 if the caller was woken up. Note that a wake-up can > also be caused by common futex usage patterns in unrelated code > that happened to have previously used the futex word's memory > location (e.g., typical futex-based implementations of Pthreads > mutexes can cause this under some conditions). Therefore, call‐ > ers should always conservatively assume that a return value of 0 > can mean a spurious wake-up, and use the futex word's value > (i.e., the user-space synchronization scheme) to decide whether > to continue to block or not. > > I'm pretty sure this is what is happening here. Here is the series of patches for review on gerrit: remote: https://review.lttng.org/c/userspace-rcu/+/8441 Fix: workqueue: futex wait: handle spurious futex wakeups [NEW] remote: https://review.lttng.org/c/userspace-rcu/+/8442 Fix: urcu: futex wait: handle spurious futex wakeups [NEW] remote: https://review.lttng.org/c/userspace-rcu/+/8443 Fix: call_rcu: futex wait: handle spurious futex wakeups [NEW] remote: https://review.lttng.org/c/userspace-rcu/+/8444 Fix: urcu-wait: futex wait: handle spurious futex wakeups [NEW] remote: https://review.lttng.org/c/userspace-rcu/+/8445 Fix: defer_rcu: futex wait: handle spurious futex wakeups [NEW] remote: https://review.lttng.org/c/userspace-rcu/+/8446 Fix: urcu-qsbr: futex wait: handle spurious futex wakeups [NEW] Thanks, Mathieu > > Thanks, > > Mathieu > > >> >> 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 _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
