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

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Message-ID: <22f99ee1d9b245c2a356d4d555b54e6a@AcuMS.aculab.com>
Date:   Fri, 30 Oct 2020 22:16:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Benjamin Segall' <bsegall@...gle.com>, Hui Su <sh_def@....com>
CC:     "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

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.

	avid

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