Re: [PATCH 3/3] powerpc/pseries/cpuhp: delete add/remove_by_count code

Daniel Henrique Barboza [off-list ref] writes:

On 9/20/21 10:55, Nathan Lynch wrote:

quoted

The core DLPAR code supports two actions (add and remove) and three
subtypes of action:

* By DRC index: the action is attempted on a single specified resource.
   This is the usual case for processors.
* By indexed count: the action is attempted on a range of resources
   beginning at the specified index. This is implemented only by the memory
   DLPAR code.
* By count: the lower layer (CPU or memory) is responsible for locating the
   specified number of resources to which the action can be applied.

I cannot find any evidence of the "by count" subtype being used by drmgr or
qemu for processors. And when I try to exercise this code, the add case
does not work:

Just to clarify: did you check both CPU and memory cases and found out that the
'by count' subtype isn't used with CPUs, but drmgr has some cases in which
'by count' is used with LMBs?

Yes, drmgr uses both the 'by count' and the 'by index' methods for
memory currently on PowerVM.

I'm asking because I worked with a part of the LMB removal code a few months ago,
and got stuck in a situation in which the 'by count' and 'by indexed count' are
similar enough to feel repetitive, but distinct enough to not be easily reduced
into a single function. If drmgr wasn't using the 'by count' subtypes for LMBs
that would be a good chance for more code redux.

The 'by count' method is definitely used for memory on PowerVM. I was
under the impression that the 'by indexed count' method was used by qemu
for memory sometimes; I'm pretty sure it's not used on PowerVM.

quoted

Summary:

* This code has not worked reliably since its introduction.
* There is no evidence that it is used.
* It contains questionable rollback behaviors in error paths which are
   difficult to test.

So let's remove it.

Signed-off-by: Nathan Lynch <redacted>
Fixes: ac71380071d1 ("powerpc/pseries: Add CPU dlpar remove functionality")
Fixes: 90edf184b9b7 ("powerpc/pseries: Add CPU dlpar add functionality")
Fixes: b015f6bc9547 ("powerpc/pseries: Add cpu DLPAR support for drc-info property")
---

Tested with a QEMU pseries guest, no issues found.


Reviewed-by: Daniel Henrique Barboza <redacted>
Tested-by: Daniel Henrique Barboza <redacted>

Thanks!