Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Date:   Sun, 24 Sep 2023 11:29:01 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org, mgorman@suse.de,
        rostedt@goodmis.org, jon.grimm@amd.com, bharata@amd.com,
        raghavendra.kt@amd.com, boris.ostrovsky@oracle.com,
        konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
Message-ID: <ZRAPbXk4N2dntgyA@casper.infradead.org>
References: <CAHk-=whMkp68vNxVn1H3qe_P7n=X2sWPL9kvW22dsvMFH8FcQQ@mail.gmail.com>
 <20230911150410.GC9098@noisy.programming.kicks-ass.net>
 <87h6o01w1a.fsf@oracle.com>
 <20230912082606.GB35261@noisy.programming.kicks-ass.net>
 <87cyyfxd4k.ffs@tglx>
 <CAHk-=whnwC01m_1f-gaM1xbvvwzwTiKitrWniA-ChZv+bM03dg@mail.gmail.com>
 <87led2wdj0.ffs@tglx>
 <87h6nkh5bw.ffs@tglx>
 <ZQ/i6CC86PzaYCzw@casper.infradead.org>
 <878r8wgg3b.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <878r8wgg3b.ffs@tglx>
Precedence: bulk

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);