tsacct: skip all kernel threads

This patch is a preparation step for HPCC, for the OOM killer
improvements.  I suspect that this patch is useful on its own, because it
really makes no sense to sum up accounting statistics of use_mm within
kernel threads which are only temporarily using those mm.

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 when introducing the new hierarchical percpu counters to
replace RSS counters, which requires a pointer dereference when reading
the approximate counter sum.  The current percpu counters simply load a
zeroed atomic counter, which happen to work.

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

This is a preparation step before introducing the hierarchical percpu
counters.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mathieu Desnoyers <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Aboorva Devarajan <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Christan König <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Liam R . Howlett" <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Martin Liu <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Sweet Tea Dorminy <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: compudj

kernel/tsacct.c

@@ -125,7 +125,7 @@ static void __acct_update_integrals(struct task_struct *tsk,
 {
 	u64 time, delta;
 
-	if (!likely(tsk->mm))
+	if (unlikely(!tsk->mm || (tsk->flags & PF_KTHREAD)))
 		return;
 
 	time = stime + utime;