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

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Date:   Mon, 2 Nov 2020 22:10:46 +0800
From:   Hui Su <sh_def@....com>
To:     Phil Auld <pauld@...hat.com>, David.Laight@...lab.com,
        linux-kernel@...r.kernel.org, bsegall@...gle.com, mingo@...hat.com
Subject: Re: [PATCH] sched/fair: remove the spin_lock operations

On Mon, Nov 02, 2020 at 08:53:41AM -0500, Phil Auld wrote:
> 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
> 

Yeah, thanks for your explanation, Phil.

It is just a leftover.