Messages in this thread |  | | Date | Wed, 26 Sep 2018 10:04:38 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [Problem] Cache line starvation |
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On Wed, 26 Sep 2018, Peter Zijlstra wrote: > On Fri, Sep 21, 2018 at 02:02:26PM +0200, Sebastian Andrzej Siewior wrote: > > 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. > > I've tried really hard to reproduce this in userspace, but so far have > not had any luck. Looks to be a real tricky thing to make happen.
It's probably equally tricky to write a reproducer as it was to instrument the thing. I assume it's a combination of code sequences on both CPUs which involve other (unrelated) lock instructions on the way.
Thanks,
tglx
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