Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

Matthew Wilcox
2023-09-24T10:29:16Z

On Sun, Sep 24, 2023 at 09:55:52AM +0200, Thomas Gleixner wrote:
> On Sun, Sep 24 2023 at 08:19, Matthew Wilcox wrote:
> > On Sun, Sep 24, 2023 at 12:50:43AM +0200, Thomas Gleixner wrote:
> >> cond_resched() cannot nest and is obviously scope-less.
> >>
> >> The TIF_ALLOW_RESCHED mechanism, which sparked this discussion only
> >> pretends to be scoped.
> >>
> >> As Peter pointed out it does not properly nest with other mechanisms and
> >> it cannot even nest in itself because it is boolean.
> >
> > We can nest a single bit without turning it into a counter -- we
> > do this for memalloc_nofs_save() for example. Simply return the
> > current value of the bit, and pass it to _restore().
>
> Right.
>
> That works, but the reverse logic still does not make sense:
>
> allow_resched();
> ....
> spin_lock();
>
> while
> resched_now_is_suboptimal();
> ...
> spin_lock();
>
> works.

Oh, indeed. I had in mind

state = resched_now_is_suboptimal();
spin_lock();
...
spin_unlock();
resched_might_be_optimal_again(state);

... or we could bundle it up ...

state = spin_lock_resched_disable();
...
spin_unlock_resched_restore(state);