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 20:36:17 +1100 (AEDT)
From:	Michael Ellerman <mpe@...erman.id.au>
To:	"Shreyas B. Prabhu" <shreyas@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Cc:	"Shreyas B. Prabhu" <shreyas@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [v3, 1/3] powerpc: Fix cpu_online_cores_map to return only online threads mask

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?

> 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.

cheers
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