Re: [PATCH] memcg: remove unneeded preempt_disable

[Johannes Weiner <jweiner@redhat.com>]

On Tue, Sep 06, 2011 at 07:04:24PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 6 Sep 2011 11:58:52 +0200
> Johannes Weiner <jweiner@redhat.com> wrote:
> 
> > On Wed, Aug 17, 2011 at 11:50:53PM -0700, Greg Thelen wrote:
> > > Both mem_cgroup_charge_statistics() and mem_cgroup_move_account() were
> > > unnecessarily disabling preemption when adjusting per-cpu counters:
> > >     preempt_disable()
> > >     __this_cpu_xxx()
> > >     __this_cpu_yyy()
> > >     preempt_enable()
> > > 
> > > This change does not disable preemption and thus CPU switch is possible
> > > within these routines.  This does not cause a problem because the total
> > > of all cpu counters is summed when reporting stats.  Now both
> > > mem_cgroup_charge_statistics() and mem_cgroup_move_account() look like:
> > >     this_cpu_xxx()
> > >     this_cpu_yyy()
> > > 
> > > Reported-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > Signed-off-by: Greg Thelen <gthelen@google.com>
> > 
> > I just noticed that both cases have preemption disabled anyway because
> > of the page_cgroup bit spinlock.
> > 
> > So removing the preempt_disable() is fine but we can even keep the
> > non-atomic __this_cpu operations.
> > 
> > Something like this instead?
> > 
> > ---
> > From: Johannes Weiner <jweiner@redhat.com>
> > Subject: mm: memcg: remove needless recursive preemption disabling
> > 
> > Callsites of mem_cgroup_charge_statistics() hold the page_cgroup bit
> > spinlock, which implies disabled preemption.
> > 
> > The same goes for the explicit preemption disabling to account mapped
> > file pages in mem_cgroup_move_account().
> > 
> > The explicit disabling of preemption in both cases is redundant.
> > 
> > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
> 
> Could you add comments as
> "This operation is called under bit spin lock !" ?

I left it as is in the file-mapped case, because if you read the
__this_cpu ops and go looking for who disables preemption, you see the
lock_page_cgroup() immediately a few lines above.

But I agree that the charge_statistics() is much less obvious, so I
added a line there.

Is that okay?

> Nice catch.
> 
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hioryu@jp.fujitsu.com>

Thanks!

---

Signed-off-by: Johannes Weiner <jweiner@redhat.com>
---
 mm/memcontrol.c |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -615,6 +615,7 @@ static unsigned long mem_cgroup_read_eve
 	return val;
 }
 
+/* Must be called with preemption disabled */
 static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
 					 bool file, int nr_pages)
 {