From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932318Ab3AYTUw (ORCPT ); Fri, 25 Jan 2013 14:20:52 -0500 Received: from mx1.redhat.com ([209.132.183.28]:53585 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757481Ab3AYTUk (ORCPT ); Fri, 25 Jan 2013 14:20:40 -0500 Date: Fri, 25 Jan 2013 14:19:17 -0500 From: Rik van Riel To: linux-kernel@vger.kernel.org Cc: aquini@redhat.com, walken@google.com, eric.dumazet@gmail.com, lwoodman@redhat.com, knoel@redhat.com, chegu_vinod@hp.com, raghavendra.kt@linux.vnet.ibm.com, mingo@redhat.com Subject: [PATCH -v4 5/5] x86,smp: limit spinlock delay on virtual machines Message-ID: <20130125141917.6d5960a8@annuminas.surriel.com> In-Reply-To: <20130125140553.060b8ced@annuminas.surriel.com> References: <20130125140553.060b8ced@annuminas.surriel.com> Organization: Red Hat, Inc. Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Modern Intel and AMD CPUs will trap to the host when the guest is spinning on a spinlock, allowing the host to schedule in something else. This effectively means the host is taking care of spinlock backoff for virtual machines. It also means that doing the spinlock backoff in the guest anyway can lead to totally unpredictable results, extremely large backoffs, and performance regressions. To prevent those problems, we limit the spinlock backoff delay, when running in a virtual machine, to a small value. Signed-off-by: Rik van Riel --- arch/x86/include/asm/processor.h | 2 ++ arch/x86/kernel/setup.c | 2 ++ arch/x86/kernel/smp.c | 30 ++++++++++++++++++++++++------ 3 files changed, 28 insertions(+), 6 deletions(-) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index 888184b..a365f97 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -997,6 +997,8 @@ extern bool cpu_has_amd_erratum(const int *); extern unsigned long arch_align_stack(unsigned long sp); extern void free_init_pages(char *what, unsigned long begin, unsigned long end); +extern void init_spinlock_delay(void); + void default_idle(void); bool set_pm_idle_to_default(void); diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c index 23ddd55..b834eae 100644 --- a/arch/x86/kernel/setup.c +++ b/arch/x86/kernel/setup.c @@ -1048,6 +1048,8 @@ void __init setup_arch(char **cmdline_p) arch_init_ideal_nops(); + init_spinlock_delay(); + register_refined_jiffies(CLOCK_TICK_RATE); #ifdef CONFIG_EFI diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c index 1877890..b1a65f0 100644 --- a/arch/x86/kernel/smp.c +++ b/arch/x86/kernel/smp.c @@ -31,6 +31,7 @@ #include #include #include +#include /* * Some notes on x86 processor bugs affecting SMP operation: * @@ -114,6 +115,27 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1); static bool smp_no_nmi_ipi = false; /* + * Modern Intel and AMD CPUs tell the hypervisor when a guest is + * spinning excessively on a spinlock. The hypervisor will then + * schedule something else, effectively taking care of the backoff + * for us. Doing our own backoff on top of the hypervisor's pause + * loop exit handling can lead to excessively long delays, and + * performance degradations. Limit the spinlock delay in virtual + * machines to a smaller value. + */ +#define DELAY_SHIFT 8 +#define DELAY_FIXED_1 (1<> DELAY_SHIFT;