mm: hugetlb: improve parallel huge page allocation time

mm: hugetlb: improve parallel huge page allocation time

Patch series "Add a command line option that enables control of how many
threads should be used to allocate huge pages", v2.

Allocating huge pages can take a very long time on servers with terabytes
of memory even when they are allocated at boot time where the allocation
happens in parallel.

Before this series, the kernel used a hard coded value of 2 threads per
NUMA node for these allocations.  This value might have been good enough
in the past but it is not sufficient to fully utilize newer systems.

This series changes the default so the kernel uses 25% of the available
hardware threads for these allocations.  In addition, we allow the user
that wish to micro-optimize the allocation time to override this value via
a new kernel parameter.

We tested this on 2 generations of Xeon CPUs and the results show a big
improvement of the overall allocation time.

+-----------------------+-------+-------+-------+-------+-------+
| threads               |   8   |   16  |   32  |   64  |   128 |
+-----------------------+-------+-------+-------+-------+-------+
| skylake      144 cpus |   44s |   22s |   16s |   19s |   20s |
| cascade lake 192 cpus |   39s |   20s |   11s |   10s |    9s |
+-----------------------+-------+-------+-------+-------+-------+

On skylake, we see an improvment of 2.75x when using 32 threads, on
cascade lake we can get even better at 4.3x when we use 128 threads.

This speedup is quite significant and users of large machines like these
should have the option to make the machines boot as fast as possible.

This patch (of 3):

Before this patch, the kernel currently used a hard coded value of 2
threads per NUMA node for these allocations.

This patch changes this policy and the kernel now uses 25% of the
available hardware threads for the allocations.

Link: https://lkml.kernel.org/r/20250227-hugepage-parameter-v2-0-7db8c6dc0453@cyberus-technology.de
Link: https://lkml.kernel.org/r/20250227-hugepage-parameter-v2-1-7db8c6dc0453@cyberus-technology.de
Signed-off-by: Thomas Prescher <thomas.prescher@cyberus-technology.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>