linux-kernel - Re: [v3, 1/3] powerpc: Fix cpu_online_cores_map to return only online threads mask

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Date:	Mon, 30 Mar 2015 22:30:51 +0530
From:	Shreyas B Prabhu <shreyas@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>, linux-kernel@...r.kernel.org
CC:	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [v3, 1/3] powerpc: Fix cpu_online_cores_map to return only online
 threads mask



On Monday 30 March 2015 03:06 PM, Michael Ellerman wrote:
> On Sun, 2015-22-03 at 04:42:57 UTC, "Shreyas B. Prabhu" wrote:
>> Currently, cpu_online_cores_map returns a mask, which for every core
>> that has atleast one online thread, has the first-cpu-of-that-core's bit
>> set. 
> 
>   ... which for every core with at least one online thread, has the bit for
>   thread 0 of the core set to 1, and the bits for all other threads of the core
>   set to 0.
> 
> Maybe that's clearer?
> 
>> But the first cpu itself may not be online always. In such cases, if
>                    ^
> 		   of the core
> 
>> the returned mask is used for IPI, then it'll cause IPIs to be skipped
>> on cores where the first thread is offline.
> 
>   .. because the IPI code refuses to send IPIs to offline threads, right?

Yes.
> 
>> Fix this by setting first-online-cpu-of-the-core's bit in the mask.
> 
>   .. by setting the bit of the first online thread in the core.
> 
>> This is done by fixing this in the underlying function
>> cpu_thread_mask_to_cores.
> 
> 
> The result has the property that for all cores with online threads, there is
> one bit set in the returned map. And further, all bits that are set in the
> returned map correspond to online threads.
> 
> 
>> Signed-off-by: Shreyas B. Prabhu <shreyas@...ux.vnet.ibm.com>
>> ---
>> This patch is new in v3
>>
>> In an example scenario where all the threads of 1st core are offline
>> and argument to cpu_thread_mask_to_cores is cpu_possible_mask,
>> with this implementation, return value will not have any bit
>> corresponding to 1st core set. I think that should be okay. Any thoughts?
> 
> Looking at linux-next:
> 
>   $ git grep cpu_thread_mask_to_cores
>   arch/powerpc/include/asm/cputhreads.h:/* cpu_thread_mask_to_cores - Return a cpumask of one per cores
>   arch/powerpc/include/asm/cputhreads.h:static inline cpumask_t cpu_thread_mask_to_cores(const struct cpumask *threads)
>   arch/powerpc/include/asm/cputhreads.h:  return cpu_thread_mask_to_cores(cpu_online_mask);
>   $ git grep cpu_online_cores_map
>   arch/powerpc/include/asm/cputhreads.h:static inline cpumask_t cpu_online_cores_map(void)
> 
> ie. There are no users.
> 
> So yeah I think we can change the semantics of this, and the semantics you
> describe make sense.
> 
> If you agree with my changelog comments I'm happy to fix that up and merge
> this, or you can send a v4 if you like.
> 

I'll fix the changelog in v4.
> cheers
> 

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