From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1754881AbdDNUhR (ORCPT <rfc822;w@1wt.eu>);
        Fri, 14 Apr 2017 16:37:17 -0400
Received: from Galois.linutronix.de ([146.0.238.70]:43302 "EHLO
        Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751518AbdDNUhO (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 14 Apr 2017 16:37:14 -0400
Date: Fri, 14 Apr 2017 22:36:56 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Chen Yu <yu.c.chen@intel.com>
cc: Prarit Bhargava <prarit@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Len Brown <len.brown@intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/irq: Do not check available vectors if current CPU
 has no irq to migrate
In-Reply-To: <20170410155151.GA9186@yu-desktop-1.sh.intel.com>
Message-ID: <alpine.DEB.2.20.1704142232280.2327@nanos>
References: <1491746031-31107-1-git-send-email-yu.c.chen@intel.com> <58EB6BE7.6020802@redhat.com> <20170410155151.GA9186@yu-desktop-1.sh.intel.com>
User-Agent: Alpine 2.20 (DEB 67 2015-01-07)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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;
+			}
 		}
 	}