linux-kernel - Re: [RFC patch] Use IPI_shortcut for lapic timer broadcast

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Message-Id: <1246580583.4534.32443.camel@localhost.localdomain>
Date:	Thu, 02 Jul 2009 17:23:03 -0700
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	Luming Yu <luming.yu@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC patch] Use IPI_shortcut for lapic timer broadcast

On Tue, 2009-06-30 at 00:01 -0700, Luming Yu wrote:
> Thanks for review. How about the following patch?
> 
> /**The patch is enclosed in text attachment**
> **Using web client to send the patch**
> **below is for review, please apply attached  patch*/
> 
> 
> Signed-off-by: Yu Luming <luming.yu@...el.com>
> 
>  apic.c         |    5 ++++-
>  apic_flat_64.c |    7 ++++++-
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> 
> --- linux-2.6.30-rc6/arch/x86/kernel/apic/apic.c.0	2009-06-29
> 23:45:05.000000000 -0600
> +++ linux-2.6.30-rc6/arch/x86/kernel/apic/apic.c	2009-06-30
> 00:37:56.000000000 -0600
> @@ -419,7 +419,10 @@
>  static void lapic_timer_broadcast(const struct cpumask *mask)
>  {
>  #ifdef CONFIG_SMP
> -	apic->send_IPI_mask(mask, LOCAL_TIMER_VECTOR);
> +	if (unlikely(cpumask_weight(mask) == num_online_cpus() -1))
> +		apic->send_IPI_all(LOCAL_TIMER_VECTOR);
> +	else
> +		apic->send_IPI_mask(mask, LOCAL_TIMER_VECTOR);
>  #endif
>  }

I dont think it is a good idea to pay the penalty for cpumask_weight and
num_online_cpus, for the more common tickless case to fix this problem
with less common no tickeless case.

We should be able to add an interface to get tick_nohz_enabled from
tick-sched.c and use that instead.

Thanks,
Venki

> 
> --- linux-2.6.30-rc6/arch/x86/kernel/apic/apic_flat_64.c.0	2009-06-29
> 00:13:26.000000000 -0600
> +++ linux-2.6.30-rc6/arch/x86/kernel/apic/apic_flat_64.c	2009-06-29
> 00:11:23.000000000 -0600
> @@ -274,7 +274,12 @@
> 
>  static void physflat_send_IPI_all(int vector)
>  {
> -	physflat_send_IPI_mask(cpu_online_mask, vector);
> +	if (vector == NMI_VECTOR) {
> +		physflat_send_IPI_mask(cpu_online_mask, vector);
> +	} else {
> +		__default_send_IPI_shortcut(APIC_DEST_ALLINC,
> +					    vector, apic->dest_logical);
> +	}
>  }
> 
>  static unsigned int physflat_cpu_mask_to_apicid(const struct cpumask *cpumask)

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