When we hit acct_account_cputime within a irq handler over a kthread
that happens to use a userspace mm, we end up summing up the mm's RSS
into the tsk acct_rss_mem1, which eventually decays.

I don't see a good rationale behind tracking the mm's rss in that way
when a kthread use a userspace mm temporarily through use_mm.

It causes issues with init_mm and efi_mm which only partially initialize
their mm_struct.

Skip all kernel threads in acct_account_cputime(), not just those that
happen to have a NULL mm.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-mm@kvack.org
---
 kernel/tsacct.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 6ea2f6363b90..3ef149b1245d 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -125,7 +125,7 @@ static void __acct_update_integrals(struct task_struct *tsk,
 {
 	u64 time, delta;
 
-	if (!likely(tsk->mm))
+	if (!tsk->mm || (tsk->flags & PF_KTHREAD))
 		return;
 
 	time = stime + utime;
-- 
2.39.5