Re: [PATCH RT] Defer migrate_enable migration while task state != TASK_RUNNING

[Julia Cartwright]

Hey Joe-

Thanks for the writeup.

On Fri, Mar 23, 2018 at 11:09:59AM -0400, joe.korty@xxxxxxxxxxxxxxxxx wrote:
> I see the below kernel splat in 4.9-rt when I run a test program that
> continually changes the affinity of some set of running pids:
> 
>    do not call blocking ops when !TASK_RUNNING; state=2 set at ...
>       ...
>       stop_one_cpu+0x60/0x80
>       migrate_enable+0x21f/0x3e0
>       rt_spin_unlock+0x2f/0x40
>       prepare_to_wait+0x5c/0x80
>       ...

This is clearly a problem.

> The reason is that spin_unlock, write_unlock, and read_unlock call
> migrate_enable, and since 4.4-rt, migrate_enable will sleep if it discovers
> that a migration is in order.  But sleeping in the unlock services is not
> expected by most kernel developers,

I don't buy this, see below:

> and where that counts most is in code sequences like the following:
>
>   set_current_state(TASK_UNINTERRUPIBLE);
>   spin_unlock(&s);
>   schedule();

The analog in mainline is CONFIG_PREEMPT and the implicit
preempt_enable() in spin_unlock().  In this configuration, a kernel
developer should _absolutely_ expect their task to be suspended (and
potentially migrated), _regardless of the task state_ if there is a
preemption event on the CPU on which this task is executing.

Similarly, on RT, there is nothing _conceptually_ wrong on RT with
migrating on migrate_enable(), regardless of task state, if there is a
pending migration event.

It's clear, however, that the mechanism used here is broken ...

   Julia