[Devel] Re: [PATCH -mmotm 3/3] memcg: dirty pages instrumentation

[Andrea Righi]

On Wed, Mar 03, 2010 at 12:47:03PM +0100, Andrea Righi wrote:
> On Tue, Mar 02, 2010 at 06:59:32PM -0500, Vivek Goyal wrote:
> > On Tue, Mar 02, 2010 at 11:22:48PM +0100, Andrea Righi wrote:
> > > On Tue, Mar 02, 2010 at 10:05:29AM -0500, Vivek Goyal wrote:
> > > > On Mon, Mar 01, 2010 at 11:18:31PM +0100, Andrea Righi wrote:
> > > > > On Mon, Mar 01, 2010 at 05:02:08PM -0500, Vivek Goyal wrote:
> > > > > > > @@ -686,10 +699,14 @@ void throttle_vm_writeout(gfp_t gfp_mask)
> > > > > > >                   */
> > > > > > >                  dirty_thresh += dirty_thresh / 10;      /* wheeee... */
> > > > > > >  
> > > > > > > -                if (global_page_state(NR_UNSTABLE_NFS) +
> > > > > > > -			global_page_state(NR_WRITEBACK) <= dirty_thresh)
> > > > > > > -                        	break;
> > > > > > > -                congestion_wait(BLK_RW_ASYNC, HZ/10);
> > > > > > > +
> > > > > > > +		dirty = mem_cgroup_page_stat(MEMCG_NR_DIRTY_WRITEBACK_PAGES);
> > > > > > > +		if (dirty < 0)
> > > > > > > +			dirty = global_page_state(NR_UNSTABLE_NFS) +
> > > > > > > +				global_page_state(NR_WRITEBACK);
> > > > > > 
> > > > > > dirty is unsigned long. As mentioned last time, above will never be true?
> > > > > > In general these patches look ok to me. I will do some testing with these.
> > > > > 
> > > > > Re-introduced the same bug. My bad. :(
> > > > > 
> > > > > The value returned from mem_cgroup_page_stat() can be negative, i.e.
> > > > > when memory cgroup is disabled. We could simply use a long for dirty,
> > > > > the unit is in # of pages so s64 should be enough. Or cast dirty to long
> > > > > only for the check (see below).
> > > > > 
> > > > > Thanks!
> > > > > -Andrea
> > > > > 
> > > > > Signed-off-by: Andrea Righi <arighi at develer.com>
> > > > > ---
> > > > >  mm/page-writeback.c |    2 +-
> > > > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > > > 
> > > > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > > > index d83f41c..dbee976 100644
> > > > > --- a/mm/page-writeback.c
> > > > > +++ b/mm/page-writeback.c
> > > > > @@ -701,7 +701,7 @@ void throttle_vm_writeout(gfp_t gfp_mask)
> > > > >  
> > > > >  
> > > > >  		dirty = mem_cgroup_page_stat(MEMCG_NR_DIRTY_WRITEBACK_PAGES);
> > > > > -		if (dirty < 0)
> > > > > +		if ((long)dirty < 0)
> > > > 
> > > > This will also be problematic as on 32bit systems, your uppper limit of
> > > > dirty memory will be 2G?
> > > > 
> > > > I guess, I will prefer one of the two.
> > > > 
> > > > - return the error code from function and pass a pointer to store stats
> > > >   in as function argument.
> > > > 
> > > > - Or Peter's suggestion of checking mem_cgroup_has_dirty_limit() and if
> > > >   per cgroup dirty control is enabled, then use per cgroup stats. In that
> > > >   case you don't have to return negative values.
> > > > 
> > > >   Only tricky part will be careful accouting so that none of the stats go
> > > >   negative in corner cases of migration etc.
> > > 
> > > What do you think about Peter's suggestion + the locking stuff? (see the
> > > previous email). Otherwise, I'll choose the other solution, passing a
> > > pointer and always return the error code is not bad.
> > > 
> > 
> > Ok, so you are worried about that by the we finish mem_cgroup_has_dirty_limit()
> > call, task might change cgroup and later we might call
> > mem_cgroup_get_page_stat() on a different cgroup altogether which might or
> > might not have dirty limits specified?
> 
> Correct.
> 
> > 
> > But in what cases you don't want to use memory cgroup specified limit? I
> > thought cgroup disabled what the only case where we need to use global
> > limits. Otherwise a memory cgroup will have either dirty_bytes specified
> > or by default inherit global dirty_ratio which is a valid number. If
> > that's the case then you don't have to take rcu_lock() outside
> > get_page_stat()?
> > 
> > IOW, apart from cgroup being disabled, what are the other cases where you
> > expect to not use cgroup's page stat and use global stats?
> 
> At boot, when mem_cgroup_from_task() may return NULL. But this is not
> related to the RCU acquisition.

Nevermind. You're right. In any case even if a task is migrated to a
different cgroup it will always have mem_cgroup_has_dirty_limit() ==
true.

So RCU protection is not needed outside these functions.

OK, I'll go with the Peter's suggestion.

Thanks!
-Andrea
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