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: <20150721004104.GN3717@linux.vnet.ibm.com>
Date:	Mon, 20 Jul 2015 17:41:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Joe Perches <joe@...ches.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, Jul 17, 2015 at 03:37:54PM -0700, Joe Perches wrote:
> 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*\(/) {

Fair enough, but this pattern is used rather heavily in this script,
and I don't trust my perl enough to want to create a new pattern.

I see a couple of possible ways forward:

1.	I hand this patch off to you, and you update it as you deem fit.

2.	I push the patch as is (after the fix below) and you update
	checkpatch as a whole to use the more efficient code.

How would you like to proceed?

> > +			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.

I did make this change.

								Thanx, Paul

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