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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
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Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
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Date:   Sun, 24 Sep 2023 09:55:52 +0200
Message-ID: <878r8wgg3b.ffs@tglx>
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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