[PATCH v2 5/6] sched/cache: Skip kernel thread for cache aware scheduling

[Tim Chen]

From: Chen Yu <yu.c.chen@xxxxxxxxx>

Kernel thread should not be covered by cache aware scheduling as
it borrows the statistics from the user space thread. Filter the
kernel thread in account_mm_sched().

In theory a kernel thread does not have any valid
cache group, so !grp should gate the kernel thread.
Add the PF_KTHREAD check explicitly here for safety
reasons, to guard against future modifications and
to pair with task_tick_cache().

Fixes: df0d98475954 ("sched/cache: Introduce infrastructure for cache-aware load balancing")
Cc: stable@xxxxxxxxxx #7.2.x
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3e2d236be2a3..341d2f9ed72b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1805,8 +1805,14 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
 	/*
 	 * init_task, kthreads and user thread created
 	 * by user_mode_thread() don't have a cache group.
-	 */
-	if (!grp || !grp->pcpu_sched)
+	 * In theory a kernel thread does not have any valid
+	 * cache group, because sched_cache_fork() is not
+	 * invoked for a kernel thread - !grp should gate the
+	 * kernel thread. Add the PF_KTHREAD check explicitly
+	 * here for safety reasons, to guard against future
+	 * modifications and to pair with task_tick_cache().
+	 */
+	if (!grp || p->flags & PF_KTHREAD || !grp->pcpu_sched)
 		return;
 
 	pcpu_sched = per_cpu_ptr(grp->pcpu_sched, cpu_of(rq));
-- 
2.32.0