linux-kernel - Re: [PATCH] x86/irq: Do not check available vectors if current CPU has no irq to migrate

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Date:   Fri, 14 Apr 2017 22:36:56 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Chen Yu <yu.c.chen@...el.com>
cc:     Prarit Bhargava <prarit@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <len.brown@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/irq: Do not check available vectors if current CPU
 has no irq to migrate

On Mon, 10 Apr 2017, Chen Yu wrote:
> I've just done a comparison of the function duration of
> check_irq_vectors_for_cpu_disable() with/without the patch
> (32 online CPUs)
> before patch:
> [001] d..1   264.017803: check_irq_vectors_for_cpu_disable <-native_cpu_disable
> [001] d..1   264.017886: clear_local_APIC <-native_cpu_disable
> after patch:
> [001] d..1   862.011847: check_irq_vectors_for_cpu_disable <-native_cpu_disable
> [001] d..1   862.011850: clear_local_APIC <-native_cpu_disable
> About  (83 - 17) = 66 microsecond.

Not really impressive if you subtract the tracer overhead. Though, if you
want to micro optimize that, then you should do it proper and also break
the for_each_cpu_loop() when the count reaches the required vectors.

Thanks,

	tglx
	
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -411,8 +411,10 @@ int check_irq_vectors_for_cpu_disable(vo
 		for (vector = FIRST_EXTERNAL_VECTOR;
 		     vector < first_system_vector; vector++) {
 			if (!test_bit(vector, used_vectors) &&
-			    IS_ERR_OR_NULL(per_cpu(vector_irq, cpu)[vector]))
-			    count++;
+			    IS_ERR_OR_NULL(per_cpu(vector_irq, cpu)[vector])) {
+				if (++count == this_count)
+					return 0;
+			}
 		}
 	}