sched/numa: use hash_32 to mix up PIDs accessing VMA

before: last 6 bits of PID is used as index to store information about
tasks accessing VMA's.

after: hash_32 is used to take of cases where tasks are created over a
period of time, and thus improve collision probability.

Result:
The patch series overall improves autonuma cost.

Kernbench around more than 5% improvement and system time in mmtest
autonuma showed more than 80% improvement

Link: https://lkml.kernel.org/r/d5a9f75513300caed74e5c8570bba9317b963c2b.1677672277.git.raghavendra.kt@amd.comSigned-off-by: Raghavendra K T <raghavendra.kt@amd.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Bharata B Rao <bharata@amd.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Disha Talreja <dishaa.talreja@amd.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -1691,7 +1691,7 @@ static inline void vma_set_access_pid_bit(struct vm_area_struct *vma)
 {
 	unsigned int pid_bit;
 
-	pid_bit = current->pid % BITS_PER_LONG;
+	pid_bit = hash_32(current->pid, ilog2(BITS_PER_LONG));
 	if (vma->numab_state && !test_bit(pid_bit, &vma->numab_state->access_pids[1])) {
 		__set_bit(pid_bit, &vma->numab_state->access_pids[1]);
 	}

kernel/sched/fair.c

@@ -2941,7 +2941,7 @@ static bool vma_is_accessed(struct vm_area_struct *vma)
 		return true;
 
 	pids = vma->numab_state->access_pids[0] | vma->numab_state->access_pids[1];
-	return test_bit(current->pid % BITS_PER_LONG, &pids);
+	return test_bit(hash_32(current->pid, ilog2(BITS_PER_LONG)), &pids);
 }
 
 #define VMA_PID_RESET_PERIOD (4 * sysctl_numa_balancing_scan_delay)