On Wed, Oct 31, 2018 at 09:14:58AM +0100, Daniel Wagner wrote:
From: Daniel Wagner daniel.wagner@siemens.com
Sebastian writes:
""" We reproducibly observe cache line starvation on a Core2Duo E6850 (2 cores), a i5-6400 SKL (4 cores) and on a NXP LS2044A ARM Cortex-A72 (4 cores).
The problem can be triggered with a v4.9-RT kernel by starting
cyclictest -S -p98 -m -i2000 -b 200and as "load"
stress-ng --ptrace 4The reported maximal latency is usually less than 60us. If the problem triggers then values around 400us, 800us or even more are reported. The upperlimit is the -i parameter.
Reproduction with 4.9-RT is almost immediate on Core2Duo, ARM64 and SKL, but it took 7.5 hours to trigger on v4.14-RT on the Core2Duo.
Instrumentation show always the picture:
CPU0 CPU1 => do_syscall_64 => do_syscall_64 => SyS_ptrace => syscall_slow_exit_work => ptrace_check_attach => ptrace_do_notify / rt_read_unlock => wait_task_inactive rt_spin_lock_slowunlock() -> while task_running() __rt_mutex_unlock_common() / check_task_state() mark_wakeup_next_waiter() | raw_spin_lock_irq(&p->pi_lock); raw_spin_lock(¤t->pi_lock); | . . | raw_spin_unlock_irq(&p->pi_lock); . \ cpu_relax() .
.*IRQ* <lock acquired>In the error case we observe that the while() loop is repeated more than 5000 times which indicates that the pi_lock can be acquired. CPU1 on the other side does not make progress waiting for the same lock with interrupts disabled.
This continues until an IRQ hits CPU0. Once CPU0 starts processing the IRQ the other CPU is able to acquire pi_lock and the situation relaxes. """
This matches with the observeration for v4.4-rt on a Core2Duo E6850:
CPU 0:
- no progress for a very long time in rt_mutex_dequeue_pi):
stress-n-1931 0d..11 5060.891219: function: __try_to_take_rt_mutex stress-n-1931 0d..11 5060.891219: function: rt_mutex_dequeue stress-n-1931 0d..21 5060.891220: function: rt_mutex_enqueue_pi stress-n-1931 0....2 5060.891220: signal_generate: sig=17 errno=0 code=262148 comm=stress-ng-ptrac pid=1928 grp=1 res=1 stress-n-1931 0d..21 5060.894114: function: rt_mutex_dequeue_pi stress-n-1931 0d.h11 5060.894115: local_timer_entry: vector=239
CPU 1:
- IRQ at 5060.894114 on CPU 1 followed by the IRQ on CPU 0
stress-n-1928 1....0 5060.891215: sys_enter: NR 101 (18, 78b, 0, 0, 17, 788) stress-n-1928 1d..11 5060.891216: function: __try_to_take_rt_mutex stress-n-1928 1d..21 5060.891216: function: rt_mutex_enqueue_pi stress-n-1928 1d..21 5060.891217: function: rt_mutex_dequeue_pi stress-n-1928 1....1 5060.891217: function: rt_mutex_adjust_prio stress-n-1928 1d..11 5060.891218: function: __rt_mutex_adjust_prio stress-n-1928 1d.h10 5060.894114: local_timer_entry: vector=239
Thomas writes:
""" This has nothing to do with RT. RT is merily exposing the problem in an observable way. The same issue happens with upstream, it's harder to trigger and it's harder to observe for obvious reasons.
If you read through the discussions [see the links below] then you really see that there is an upstream issue with the x86 qrlock implementation and Peter has posted fixes which resolve it, both at the practical and the theoretical level. """
Backporting all qspinlock related patches is very likely to introduce regressions on v4.4. Therefore, the recommended solution by Peter and Thomas is to drop back to ticket spinlocks for v4.4.
Link :https://lkml.kernel.org/r/20180921120226.6xjgr4oiho22ex75@linutronix.de Link: https://lkml.kernel.org/r/20180926110117.405325143@infradead.org Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Signed-off-by: Daniel Wagner daniel.wagner@siemens.com
Thomas suggest following plan for fixing the issues on the varous stable trees:
4.4: Trivial by switching back to ticket locks.
Now queued up, thanks,
greg k-h