kernel/fork: declare max_threads __read_mostly

max_threads is initialized once by fork_init()->set_max_threads(), and
thereafter is mostly read in hot path, such as copy_process().

The fact that it is mostly read and not written to makes it candidates for
__read_mostly declarations.

I have already tested on my machine(arm64,256core,kernel-7.1.3) with
'hackbench -P -l 500 -g 100/500/1000/2000' and the results show that there
is an average of 13% improvement in performance.

before patch:
	hackbench -g    100      500      1000      2000
                    1.094    4.651    13.305    36.322

after patch:
	hackbench -g    100      500      1000      2000
                    0.823    4.487    11.107    32.987

Above data are the average values obtained from multiple tests, and there
was indeed some fluctuation in the data during the tests.

Link: https://lore.kernel.org/20260707092354.465847-1-jansef.jian@hj-micro.com
Signed-off-by: JianChunfu <jansef.jian@hj-micro.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/fork.c

@@ -143,7 +143,7 @@
 unsigned long total_forks;	/* Handle normal Linux uptimes. */
 int nr_threads;			/* The idle threads do not count.. */
 
-static int max_threads;		/* tunable limit on nr_threads */
+static int max_threads __read_mostly;		/* tunable limit on nr_threads */
 
 #define NAMED_ARRAY_INDEX(x)	[x] = __stringify(x)