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

[Thomas Gleixner <tglx@linutronix.de>]

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.

Thanks,

        tglx