Messages in this thread Patch in this message | | | Date | Wed, 11 Sep 2013 11:08:53 -0400 | From | Mathieu Desnoyers <> | Subject | [RFC PATCH] timekeeping: introduce timekeeping_is_busy() |
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Starting from commit 06c017fdd4dc48451a29ac37fc1db4a3f86b7f40 "timekeeping: Hold timekeepering locks in do_adjtimex and hardpps" (3.10 kernels), the xtime write seqlock is held across calls to __do_adjtimex(), which includes a call to notify_cmos_timer(), and hence schedule_delayed_work().
This introduces a side-effect for a set of tracepoints, including mainly the workqueue tracepoints: a tracer hooking on those tracepoints and reading current time with ktime_get() will cause hard system LOCKUP such as:
WARNING: CPU: 6 PID: 2258 at kernel/watchdog.c:245 watchdog_overflow_callback+0x93/0x9e() Watchdog detected hard LOCKUP on cpu 6 Modules linked in: lttng_probe_workqueue(O) lttng_probe_vmscan(O) lttng_probe_udp(O) lttng_probe_timer(O) lttng_probe_s] CPU: 6 PID: 2258 Comm: ntpd Tainted: G O 3.11.0 #158 Hardware name: Supermicro X7DAL/X7DAL, BIOS 6.00 12/03/2007 0000000000000000 ffffffff814f83eb ffffffff813b206a ffff88042fd87c78 ffffffff8106a07c 0000000000000000 ffffffff810c94c2 0000000000000000 ffff88041f31bc00 0000000000000000 ffff88042fd87d68 ffff88042fd87ef8 Call Trace: <NMI> [<ffffffff813b206a>] ? dump_stack+0x41/0x51 [<ffffffff8106a07c>] ? warn_slowpath_common+0x79/0x92 [<ffffffff810c94c2>] ? watchdog_overflow_callback+0x93/0x9e [<ffffffff8106a12d>] ? warn_slowpath_fmt+0x45/0x4a [<ffffffff810c94c2>] ? watchdog_overflow_callback+0x93/0x9e [<ffffffff810c942f>] ? watchdog_enable_all_cpus.part.2+0x31/0x31 [<ffffffff810ecc66>] ? __perf_event_overflow+0x12c/0x1ae [<ffffffff810eab60>] ? perf_event_update_userpage+0x13/0xc2 [<ffffffff81016820>] ? intel_pmu_handle_irq+0x26a/0x2fd [<ffffffff813b7a0b>] ? perf_event_nmi_handler+0x24/0x3d [<ffffffff813b728f>] ? nmi_handle.isra.3+0x58/0x12f [<ffffffff813b7a59>] ? perf_ibs_nmi_handler+0x35/0x35 [<ffffffff813b7404>] ? do_nmi+0x9e/0x2bc [<ffffffff813b6af7>] ? end_repeat_nmi+0x1e/0x2e [<ffffffff810a2a33>] ? read_seqcount_begin.constprop.4+0x8/0xf [<ffffffff810a2a33>] ? read_seqcount_begin.constprop.4+0x8/0xf [<ffffffff810a2a33>] ? read_seqcount_begin.constprop.4+0x8/0xf <<EOE>> [<ffffffff810a2d6c>] ? ktime_get+0x23/0x5e [<ffffffffa0314670>] ? lib_ring_buffer_clock_read.isra.28+0x1f/0x21 [lttng_ring_buffer_client_discard] [<ffffffffa0314786>] ? lttng_event_reserve+0x112/0x3f3 [lttng_ring_buffer_client_discard] [<ffffffffa045b1c5>] ? __event_probe__workqueue_queue_work+0x72/0xe0 [lttng_probe_workqueue] [<ffffffff812ef7e9>] ? sock_aio_read.part.10+0x110/0x124 [<ffffffff81133a36>] ? do_sync_readv_writev+0x50/0x76 [<ffffffff8107d514>] ? __queue_work+0x1ab/0x265 [<ffffffff8107da7e>] ? queue_delayed_work_on+0x3f/0x4e [<ffffffff810a473d>] ? __do_adjtimex+0x408/0x413 [<ffffffff810a3e9a>] ? do_adjtimex+0x98/0xee [<ffffffff8106cec6>] ? SYSC_adjtimex+0x32/0x5d [<ffffffff813bb74b>] ? tracesys+0xdd/0xe2
LTTng uses ktime to have the same time-base across kernel and user-space, so traces gathered from LTTng-modules and LTTng-UST can be correlated. We plan on using ktime until a fast, scalable, and fine-grained time-source for tracing that can be used across kernel and user-space, and which does not rely on read seqlock for kernel-level synchronization, makes its way into the kernel.
Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Prarit Bhargava <prarit@redhat.com> Cc: John Stultz <john.stultz@linaro.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> --- diff --git a/include/linux/time.h b/include/linux/time.h index d5d229b..f2cec6b 100644 --- a/include/linux/time.h +++ b/include/linux/time.h @@ -267,4 +267,11 @@ static __always_inline void timespec_add_ns(struct timespec *a, u64 ns) a->tv_nsec = ns; } +/** + * timekeeping_is_busy - is timekeeping busy ? + * + * Returns 0 if the current execution context can safely read time. + */ +extern int timekeeping_is_busy(void); + #endif diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 48b9fff..8ec2b5f 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -34,6 +34,7 @@ static struct timekeeper timekeeper; static DEFINE_RAW_SPINLOCK(timekeeper_lock); static seqcount_t timekeeper_seq; +static int timekeeper_seq_owner = -1; static struct timekeeper shadow_timekeeper; /* flag for if timekeeping is suspended */ @@ -1690,6 +1691,9 @@ int do_adjtimex(struct timex *txc) getnstimeofday(&ts); raw_spin_lock_irqsave(&timekeeper_lock, flags); + timekeeper_seq_owner = smp_processor_id(); + /* store to timekeeper_seq_owner before seqcount */ + barrier(); write_seqcount_begin(&timekeeper_seq); orig_tai = tai = tk->tai_offset; @@ -1701,6 +1705,9 @@ int do_adjtimex(struct timex *txc) clock_was_set_delayed(); } write_seqcount_end(&timekeeper_seq); + /* store to seqcount before timekeeper_seq_owner */ + barrier(); + timekeeper_seq_owner = -1; raw_spin_unlock_irqrestore(&timekeeper_lock, flags); return ret; @@ -1715,11 +1722,17 @@ void hardpps(const struct timespec *phase_ts, const struct timespec *raw_ts) unsigned long flags; raw_spin_lock_irqsave(&timekeeper_lock, flags); + timekeeper_seq_owner = smp_processor_id(); + /* store to timekeeper_seq_owner before seqcount */ + barrier(); write_seqcount_begin(&timekeeper_seq); __hardpps(phase_ts, raw_ts); write_seqcount_end(&timekeeper_seq); + /* store to seqcount before timekeeper_seq_owner */ + barrier(); + timekeeper_seq_owner = -1; raw_spin_unlock_irqrestore(&timekeeper_lock, flags); } EXPORT_SYMBOL(hardpps); @@ -1737,3 +1750,11 @@ void xtime_update(unsigned long ticks) do_timer(ticks); write_sequnlock(&jiffies_lock); } + +int timekeeping_is_busy(void) +{ + if (timekeeper_seq_owner == raw_smp_processor_id()) + return 1; + return 0; +} +EXPORT_SYMBOL_GPL(timekeeping_is_busy); -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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