Commit 6115c793 authored Sep 07, 2015 by Peter Zijlstra Committed by Ingo Molnar Sep 13, 2015

sched/fair: Optimize __update_load_avg()



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@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 54a21385

kernel/sched/fair.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -2551,7 +2551,7 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
		{
		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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