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

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Date:	Fri, 3 Jul 2009 10:04:48 +0800
From:	Luming Yu <luming.yu@...il.com>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.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 Fri, Jul 3, 2009 at 8:23 AM, Pallipadi,
Venkatesh<venkatesh.pallipadi@...el.com> wrote:
> 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.

In NO_HZ, the lapic_timer_broadcast is rare thing too...

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

If we use send_IPI_all just for NO_HZ is disabled, we will leave a corner case
here that lapic_timer_broadcast could be really really being actively used even
NO_HZ enabled... if this will never happen, I would agree with you..

Thanks,
LUming
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