linux-kernel - Re: [PATCH] sched/fair: remove the spin

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Message-ID: <20201102135341.GA154641@lorien.usersys.redhat.com>
Date:   Mon, 2 Nov 2020 08:53:41 -0500
From:   Phil Auld <pauld@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Benjamin Segall' <bsegall@...gle.com>, Hui Su <sh_def@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "bristot@...hat.com" <bristot@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: remove the spin_lock operations

On Fri, Oct 30, 2020 at 10:16:29PM +0000 David Laight wrote:
> From: Benjamin Segall
> > Sent: 30 October 2020 18:48
> > 
> > Hui Su <sh_def@....com> writes:
> > 
> > > Since 'ab93a4bc955b ("sched/fair: Remove
> > > distribute_running fromCFS bandwidth")',there is
> > > nothing to protect between raw_spin_lock_irqsave/store()
> > > in do_sched_cfs_slack_timer().
> > >
> > > So remove it.
> > 
> > Reviewed-by: Ben Segall <bsegall@...gle.com>
> > 
> > (I might nitpick the subject to be clear that it should be trivial
> > because the lock area is empty, or call them dead or something, but it's
> > not all that important)
> 
> I don't know about this case, but a lock+unlock can be used
> to ensure that nothing else holds the lock when acquiring
> the lock requires another lock be held.
> 
> So if the normal sequence is:
> 	lock(table)
> 	# lookup item
> 	lock(item)
> 	unlock(table)
> 	....
> 	unlock(item)
> 
> Then it can make sense to do:
> 	lock(table)
> 	lock(item)
> 	unlock(item)
> 	....
> 	unlock(table)
> 
> although that ought to deserve a comment.
>

Nah, this one used to be like this :

        raw_spin_lock_irqsave(&cfs_b->lock, flags);
        lsub_positive(&cfs_b->runtime, runtime);
        cfs_b->distribute_running = 0;
        raw_spin_unlock_irqrestore(&cfs_b->lock, flags);

It's just a leftover. I agree that if it was there for some other
purpose that it would really need a comment. In this case, it's an
artifact of patch-based development I think.


Cheers,
Phil


> 	avid
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
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