linux-kernel - Re: [PATCH tip/core/rcu 15/16] scripts: Make checkpatch.pl warn on expedited RCU grace periods

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Message-ID: <1437172674.2495.60.camel@perches.com>
Date:	Fri, 17 Jul 2015 15:37:54 -0700
From:	Joe Perches <joe@...ches.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com, Andy Whitcroft <apw@...onical.com>
Subject: Re: [PATCH tip/core/rcu 15/16] scripts: Make checkpatch.pl warn on
 expedited RCU grace periods

On Fri, 2015-07-17 at 15:20 -0700, Paul E. McKenney wrote:
> The synchronize_rcu_expedited() and synchronize_sched_expedited()
> expedited-grace-period primitives induce OS jitter, which can degrade
> real-time response.  This commit therefore adds a checkpatch.pl warning
> on any patch adding them.
[]
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
[]
> +# Check for expedited grace periods that interrupt non-idle non-nohz
> +# online CPUs.  These expedited can therefore degrade real-time response
> +# if used carelessly, and should be avoided where not absolutely
> +# needed.  It is always OK to use synchronize_rcu_expedited() and
> +# synchronize_sched_expedited() at boot time (before real-time applications
> +# start) and in error situations where real-time response is compromised in
> +# any case.  Note that synchronize_srcu_expedited() does -not- interrupt
> +# other CPUs, so don't warn on uses of synchronize_srcu_expedited().
> +# Of course, nothing comes for free, and srcu_read_lock() and
> +# srcu_read_unlock() do contain full memory barriers in payment for
> +# synchronize_srcu_expedited() non-interruption properties.
> +		if ($line =~ /\b(synchronize_rcu_expedited|synchronize_sched_expedited)\(/) {

It'd be faster perl without capture groups:

		if ($line =~ /\b(?:synchronize_(?:rcu|sched)_expedited)\s*\(/) {

> +			WARN("EXPEDITED_RCU_GRACE_PERIOD",
> +			     "expedited RCU grace periods should be avoided in cases where they can degrade real-time response\n" . $herecurr);

"in cases" is rather redundant.


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