mm: hugetlb: add hugetlb_alloc_threads cmdline option

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

[akpm@linux-foundation.org: tidy up a comment]
Link: https://lkml.kernel.org/r/20250227-hugepage-parameter-v2-2-7db8c6dc0453@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>

Author: tpressure

Documentation/admin-guide/kernel-parameters.txt

@@ -1882,6 +1882,15 @@
 			Documentation/admin-guide/mm/hugetlbpage.rst.
 			Format: size[KMG]
 
+	hugepage_alloc_threads=
+			[HW] The number of threads that should be used to
+			allocate hugepages during boot. This option can be
+			used to improve system bootup time when allocating
+			a large amount of huge pages.
+			The default value is 25% of the available hardware threads.
+
+			Note that this parameter only applies to non-gigantic huge pages.
+
 	hugetlb_cma=	[HW,CMA,EARLY] The size of a CMA area used for allocation
 			of gigantic hugepages. Or using node format, the size
 			of a CMA area per node can be specified.

Documentation/admin-guide/mm/hugetlbpage.rst

@@ -145,7 +145,17 @@ hugepages
 
 	It will allocate 1 2M hugepage on node0 and 2 2M hugepages on node1.
 	If the node number is invalid,  the parameter will be ignored.
+hugepage_alloc_threads
+	Specify the number of threads that should be used to allocate hugepages
+	during boot. This parameter can be used to improve system bootup time
+	when allocating a large amount of huge pages.
 
+	The default value is 25% of the available hardware threads.
+	Example to use 8 allocation threads::
+
+		hugepage_alloc_threads=8
+
+	Note that this parameter only applies to non-gigantic huge pages.
 default_hugepagesz
 	Specify the default huge page size.  This parameter can
 	only be specified once on the command line.  default_hugepagesz can

mm/hugetlb.c

@@ -85,6 +85,7 @@ static unsigned long __initdata default_hstate_max_huge_pages;
 static bool __initdata parsed_valid_hugepagesz = true;
 static bool __initdata parsed_default_hugepagesz;
 static unsigned int default_hugepages_in_node[MAX_NUMNODES] __initdata;
+static unsigned long hugepage_allocation_threads __initdata;
 
 static char hstate_cmdline_buf[COMMAND_LINE_SIZE] __initdata;
 static int hstate_cmdline_index __initdata;
@@ -3607,8 +3608,6 @@ static unsigned long __init hugetlb_pages_alloc_boot(struct hstate *h)
 		.numa_aware	= true
 	};
 
-	unsigned int num_allocation_threads = max(num_online_cpus() / 4, 1);
-
 	job.thread_fn	= hugetlb_pages_alloc_boot_node;
 	job.start	= 0;
 	job.size	= h->max_huge_pages;
@@ -3629,9 +3628,13 @@ static unsigned long __init hugetlb_pages_alloc_boot(struct hstate *h)
 	 * | cascade lake 192 cpus |   39s |   20s |   11s |   10s |    9s |
 	 * +-----------------------+-------+-------+-------+-------+-------+
 	 */
+	if (hugepage_allocation_threads == 0) {
+		hugepage_allocation_threads = num_online_cpus() / 4;
+		hugepage_allocation_threads = max(hugepage_allocation_threads, 1);
+	}
 
-	job.max_threads	= num_allocation_threads;
-	job.min_chunk	= h->max_huge_pages / num_allocation_threads;
+	job.max_threads	= hugepage_allocation_threads;
+	job.min_chunk	= h->max_huge_pages / hugepage_allocation_threads;
 	padata_do_multithreaded(&job);
 
 	return h->nr_huge_pages;
@@ -5000,6 +5003,28 @@ void __init hugetlb_bootmem_alloc(void)
 	__hugetlb_bootmem_allocated = true;
 }
 
+/*
+ * hugepage_alloc_threads command line parsing.
+ *
+ * When set, use this specific number of threads for the boot
+ * allocation of hugepages.
+ */
+static int __init hugepage_alloc_threads_setup(char *s)
+{
+	unsigned long allocation_threads;
+
+	if (kstrtoul(s, 0, &allocation_threads) != 0)
+		return 1;
+
+	if (allocation_threads == 0)
+		return 1;
+
+	hugepage_allocation_threads = allocation_threads;
+
+	return 1;
+}
+__setup("hugepage_alloc_threads=", hugepage_alloc_threads_setup);
+
 static unsigned int allowed_mems_nr(struct hstate *h)
 {
 	int node;