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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-03-02 at 23:14 +0100, Andrea Righi wrote:
> 
> I agree mem_cgroup_has_dirty_limit() is nicer. But we must do that under
> RCU, so something like:
> 
>         rcu_read_lock();
>         if (mem_cgroup_has_dirty_limit())
>                 mem_cgroup_get_page_stat()
>         else
>                 global_page_state()
>         rcu_read_unlock();
> 
> That is bad when mem_cgroup_has_dirty_limit() always returns false
> (e.g., when memory cgroups are disabled). So I fallback to the old
> interface.

Why is it that mem_cgroup_has_dirty_limit() needs RCU when
mem_cgroup_get_page_stat() doesn't? That is, simply make
mem_cgroup_has_dirty_limit() not require RCU in the same way
*_get_page_stat() doesn't either.

> What do you think about:
> 
>         mem_cgroup_lock();
>         if (mem_cgroup_has_dirty_limit())
>                 mem_cgroup_get_page_stat()
>         else
>                 global_page_state()
>         mem_cgroup_unlock();
> 
> Where mem_cgroup_read_lock/unlock() simply expand to nothing when
> memory cgroups are disabled.

I think you're engineering the wrong way around.

> > 
> > That allows for a 0 dirty limit (which should work and basically makes
> > all io synchronous).
> 
> IMHO it is better to reserve 0 for the special value "disabled" like the
> global settings. A synchronous IO can be also achieved using a dirty
> limit of 1.

Why?! 0 clearly states no writeback cache, IOW sync writes, a 1
byte/page writeback cache effectively reduces to the same thing, but its
not the same thing conceptually. If you want to put the size and enable
into a single variable pick -1 for disable or so.