linux-kernel - Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ

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Date:	Mon, 11 Apr 2016 15:18:16 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates

On Fri, Apr 08, 2016 at 07:44:14PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 08, 2016 at 02:55:22PM +0200, Frederic Weisbecker wrote:
> > > > @@ -4540,17 +4568,8 @@ static void cpu_load_update(struct rq *this_rq, unsigned long this_load,
> > > >  
> > > >  		/* scale is effectively 1 << i now, and >> i divides by scale */
> > > >  
> > > > -		old_load = this_rq->cpu_load[i];
> > > #ifdef CONFIG_NO_HZ_COMMON
> > > > -		old_load = decay_load_missed(old_load, pending_updates - 1, i);
> > > > -		if (tickless_load) {
> > > > -			old_load -= decay_load_missed(tickless_load, pending_updates - 1, i);
> > > > -			/*
> > > > -			 * old_load can never be a negative value because a
> > > > -			 * decayed tickless_load cannot be greater than the
> > > > -			 * original tickless_load.
> > > > -			 */
> > > > -			old_load += tickless_load;
> > > > -		}
> > > #endif
> > 
> > Ah sure, if you prefer it that way, I can do that.
> 
> Yes please. I normally favour the thing you did, but here it makes
> tricky code that much harder to read.

So I tried and it warns about the unused variable tickless_load, so I
would need two scattered ifdeffery in the function:

@@ -4528,7 +4529,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
 static void cpu_load_update(struct rq *this_rq, unsigned long this_load,
 			    unsigned long pending_updates)
 {
+#ifdef CONFIG_NO_HZ_COMMON
 	unsigned long tickless_load = this_rq->cpu_load[0];
+#endif
 	int i, scale;
 
 	this_rq->nr_load_updates++;
@@ -4541,6 +4544,7 @@ static void cpu_load_update(struct rq *this_rq, unsigned long this_load,
 		/* scale is effectively 1 << i now, and >> i divides by scale */
 
 		old_load = this_rq->cpu_load[i];
+#ifdef CONFIG_NO_HZ_COMMON
 		old_load = decay_load_missed(old_load, pending_updates - 1, i);
 		if (tickless_load) {
 			old_load -= decay_load_missed(tickless_load, pending_updates - 1, i);
@@ -4551,6 +4555,7 @@ static void cpu_load_update(struct rq *this_rq, unsigned long this_load,
 			 */
 			old_load += tickless_load;
 		}
+#endif
 		new_load = this_load;
 		/*
 		 * Round up the averaging division if load is increasing. This


Are you still sure you don't want the ifdeffed inline function?

Thanks.