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

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Date:   Tue, 9 May 2017 14:54:50 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net
Subject: Re: [v3 0/9] parallelized "struct page" zeroing

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