linux-kernel - Re: [PATCH 0/6] sched/fair: Compute capacity invariant load/utilization tracking

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Date:	Mon, 7 Sep 2015 15:23:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, vincent.guittot@...aro.org,
	daniel.lezcano@...aro.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	yuyang.du@...el.com, mturquette@...libre.com, rjw@...ysocki.net,
	Juri Lelli <Juri.Lelli@....com>, sgurrappadi@...dia.com,
	pang.xunlei@....com.cn, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] sched/fair: Compute capacity invariant
 load/utilization tracking

On Mon, Sep 07, 2015 at 02:42:20PM +0200, Peter Zijlstra wrote:
> I'm of a mind to apply these patches; with two patches on top, which
> I'll post shortly.

---
Subject: sched: Optimize __update_load_avg()
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon Sep  7 15:09:15 CEST 2015

Prior to this patch; the line:

	scaled_delta_w = (delta_w * 1024) >> 10;

which is the result of the default arch_scale_freq_capacity()
function, turns into:

    1b03:	49 89 d1             	mov    %rdx,%r9
    1b06:	49 c1 e1 0a          	shl    $0xa,%r9
    1b0a:	49 c1 e9 0a          	shr    $0xa,%r9

Which is silly; when made unsigned int, GCC recognises this as
pointless ops and fails to emit them (confirmed on 4.9.3 and 5.1.1).

Furthermore, afaict unsigned is actually the correct type for these
fields anyway, as we've explicitly ruled out negative delta's earlier
in this function.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/fair.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2551,7 +2551,7 @@ __update_load_avg(u64 now, int cpu, stru
 {
 	u64 delta, scaled_delta, periods;
 	u32 contrib;
-	int delta_w, scaled_delta_w, decayed = 0;
+	unsigned int delta_w, scaled_delta_w, decayed = 0;
 	unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
 	unsigned long scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
 
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