linux-kernel - Re: [PATCH RFC 3/3] x86: use mwait for trigger API

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Message-ID: <48B6E174.7070308@goop.org>
Date:	Thu, 28 Aug 2008 10:33:40 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
CC:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH RFC 3/3] x86: use mwait for trigger API

Christian Borntraeger wrote:
> Am Samstag, 16. August 2008 schrieb Jeremy Fitzhardinge:
>
> Seems that this cant work. We never reset the t->cpus bits. That means we 
> never mwait after a kick.
>   

Yeah, I have to admit I never tested this code (it was an RFC, after
all).  And after Arjan said that mwait is unusable, I didn't spend any
more effort on it.

> See:
>
>   
>> +void mwait_trigger_reset(trigger_t *t)
>> +{
>> +	unsigned long flags;
>> +
>> +	t->trigger = 0;
>> +
>> +	local_save_flags(flags);
>> +	__get_cpu_var(mwait_saved_flags) = flags;
>> +
>> +	__monitor(&t->trigger, 0, 0);
>> +}
>> +
>> +void mwait_trigger_wait(trigger_t *t)
>> +{
>> +	unsigned long flags = __get_cpu_var(mwait_saved_flags);
>> +	int cpu = smp_processor_id();
>> +
>> +	if (irqs_disabled_flags(flags)) {
>> +		while(!cpu_isset(cpu, t->cpus)) {
>>     
>
> We check the bits here
>
>   
>> +			__mwait(0, 0);
>> +			barrier();
>> +			__monitor(&t->trigger, 0, 0);
>> +		}
>> +	} else {
>> +		while(!cpu_isset(cpu, t->cpus)) {
>>     
>
> and here
>
>   
>> +			__sti_mwait(0, 0);
>> +			barrier();
>> +			local_irq_disable();
>> +			__monitor(&t->trigger, 0, 0);
>> +		}
>> +	}
>>     

Must have lost a line somewhere.  It's supposed to clear the bit here,
before wait returns.

    J
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