Re: [v3 0/9] parallelized "struct page" zeroing

Pasha Tatashin
2017-05-09T18:54:50Z

Hi Michal,

> I like the idea of postponing the zeroing from the allocation to the
> init time. To be honest the improvement looks much larger than I would
> expect (Btw. this should be a part of the changelog rather than a
> outside link).

The improvements are larger, because this time was never measured, as
Linux does not have early boot time stamps. I added them for x86 and
SPARC to emasure the performance. I am pushing those changes through
separate patchsets.

>
> The implementation just looks too large to what I would expect. E.g. do
> we really need to add zero argument to the large part of the memblock
> API? Wouldn't it be easier to simply export memblock_virt_alloc_internal
> (or its tiny wrapper memblock_virt_alloc_core) and move the zeroing
> outside to its 2 callers? A completely untested scratched version at the
> end of the email.

I am OK, with this change. But, I do not really see a difference between:

memblock_virt_alloc_raw()
and
memblock_virt_alloc_core()

In both cases we use memblock_virt_alloc_internal(), but the only
difference is that in my case we tell memblock_virt_alloc_internal() to
zero the pages if needed, and in your case the other two callers are
zeroing it. I like moving memblock_dbg() inside
memblock_virt_alloc_internal()

>
> Also it seems that this is not 100% correct either as it only cares
> about VMEMMAP while DEFERRED_STRUCT_PAGE_INIT might be enabled also for
> SPARSEMEM. This would suggest that we would zero out pages twice,
> right?

Thank you, I will check this combination before sending out the next patch.

>
> A similar concern would go to the memory hotplug patch which will
> fall back to the slab/page allocator IIRC. On the other hand
> __init_single_page is shared with the hotplug code so again we would
> initialize 2 times.

Correct, when memory it hotplugged, to gain the benefit of this fix, and
also not to regress by actually double zeroing "struct pages" we should
not zero it out. However, I do not really have means to test it.

>
> So I suspect more changes are needed. I will have a closer look tomorrow.

Thank you for reviewing this work. I will wait for your comments before
sending out updated patches.

Pasha