linux-kernel - Re: [PATCH 1/2] kernel/sched: introduce vcpu preempted interface

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Message-Id: <5770F521.1080705@linux.vnet.ibm.com>
Date:	Mon, 27 Jun 2016 17:42:57 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	boqun.feng@...il.com
Subject: Re: [PATCH 1/2] kernel/sched: introduce vcpu preempted interface



On 2016年06月27日 16:42, Peter Zijlstra wrote:
> On Sun, Jun 26, 2016 at 06:41:54AM -0400, Pan Xinhui wrote:
>
>> +#ifdef arch_vcpu_is_preempted
>> +static inline bool vcpu_is_preempted(int cpu)
>> +{
>> +	return arch_vcpu_is_preempted(cpu);
>> +}
>> +#else
>> +static inline bool vcpu_is_preempted(int cpu)
>> +{
>> +	return 0;
>> +}
>> +#endif
>> +
>> +#ifdef arch_vcpu_get_yield_count
>> +static inline unsigned int vcpu_get_yield_count(int cpu)
>> +{
>> +	return arch_vcpu_get_yield_count(cpu);
>> +}
>> +#else
>> +static inline unsigned int vcpu_get_yield_count(int cpu)
>> +{
>> +	return 0;
>> +}
>> +#endif
>
>
> Please, just do something like:
>
> #ifndef vcpu_is_preempted
> static inline bool vcpu_is_preempted(int cpu)
> {
> 	return false;
> }
> #endif
>
> No point in making it more complicated.
>
right, vcpu_is_preempted() is good enough to handle our osq issue.

>> +static inline bool
>> +need_yield_to(int vcpu, unsigned int old_yield_count)
>
> namespace... this thing should be called: vcpu_something()
>
>> +{
>> +	/* if we find the vcpu is preempted,
>> +	* then we may want to kick it, IOW, yield to it
>> +	*/
>> +	return vcpu_is_preempted(vcpu) ||
>> +		(vcpu_get_yield_count(vcpu) != old_yield_count);
>> +}
>
> And can you make doubly sure (and mention in the Changelog) that the OSQ
> code compiles all this away when using these definitions.
>
vimdiff shows the osq_lock.o has a little difference because osq read the yield_count and prev, even they are not used.

however, as you suggest above, IF I remove the vcpu_get_yield_count() and relevant code in osq_lock,  then the binary is same.