Re: [PATCH 2/2] blk-iocost: Use alloc_percpu_gfp() to simplify the code

Hi Tejun,

Hello,

On Fri, Dec 11, 2020 at 03:13:29PM +0800, Baolin Wang wrote:

quoted

Thanks for teaching me this, at least I did not get this from the local_ops
Documentation before. Just out of curiosity, these local[64]_t variables are
also allocated from budy allocator ultimately, why they can not be
initialized to zeros on some ARCHs with __GFP_ZERO? Could you elaborate on
about this restriction? Thanks.

* It's highly unlikely but theoretically possible that some arch might need
   more than raw value to implement local semantics.

* People might wanna add debug annotations which may require more than just
   the raw value.

Thanks for your explanation. It will be helpful to add these comments 
for the code in case someone else wants to do the same thing like this 
patch in future. I can send a patch to add these comments if you have no 
objection.

quoted

By the way, seems the kyber-iosched has the same issue, since the 'struct
kyber_cpu_latency' also contains an atomic_t variable.

	kqd->cpu_latency = alloc_percpu_gfp(struct kyber_cpu_latency,
					    GFP_KERNEL | __GFP_ZERO);
	if (!kqd->cpu_latency)
		goto err_kqd;

Yeah, the lines become blurry when all existing usages are fine with zeroing
and we do end up needing to clean up those when the need arises (e.g. we
used to zero some spinlocks too). It's just a better form to stick with
initializers when they are provided.

OK. Sounds it is worth sending a patch to initialize this structure 
explicitly to keep consistent.