Re: [Problem] Cache line starvation

[Peter Zijlstra]

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(&current->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.