On 8/13/2013 10:09 AM, Linus Torvalds wrote:

On Mon, Aug 12, 2013 at 2:54 PM, Nathan Zimmer [off-list ref] wrote:

quoted

As far as extra overhead. We incur an extra function call to
ensure_page_is_initialized but that is only really expensive when we find
uninitialized pages, otherwise it is a flag check once every PTRS_PER_PMD.
To get a better feel for this we ran two quick tests.

Sorry for coming into this late and for this last version of the
patch, but I have to say that I'd *much* rather see this delayed
initialization using another data structure than hooking into the
basic page allocation ones..

I understand that you want to do delayed initialization on some TB+
memory machines, but what I don't understand is why it has to be done
when the pages have already been added to the memory management free
list.

Could we not do this much simpler: make the early boot insert the
first few gigs of memory (initialized) synchronously into the free
lists, and then have a background thread that goes through the rest?

That way the MM layer would never see the uninitialized pages.

And I bet that *nobody* cares if you "only" have a few gigs of ram
during the first few minutes of boot, and you mysteriously end up
getting more and more memory for a while until all the RAM has been
initialized.

IOW, just don't call __free_pages_bootmem() on all the pages al at
once. If we have to remove a few __init markers to be able to do some
of it later, does anybody really care?

I really really dislike this "let's check if memory is initialized at
runtime" approach.

           Linus

Initially this patch set consisted of diverting a major portion of the
memory to an "absent" list during e820 processing.  A very late initcall
was then used to dispatch a cpu per node to add that nodes's absent
memory.  By nature these ran in parallel so Nathan did the work to
"parallelize" various global resource locks to become per node locks.

This sped up insertion considerably.  And by disabling the "auto-start"
of the insertion process and using a manual start command, you could
monitor the insertion process and find hot spots in the memory
initialization code.

Also small updates to the sys/devices/{memory,node} drivers to also
display the amount of memory still "absent".

-Mike

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