[Devel] Re: [PATCH 3/5] page_cgroup: make page tracking available for blkio

[Vivek Goyal]

On Wed, Feb 23, 2011 at 12:01:47AM +0100, Andrea Righi wrote:
> On Tue, Feb 22, 2011 at 01:01:45PM -0700, Jonathan Corbet wrote:
> > On Tue, 22 Feb 2011 18:12:54 +0100
> > Andrea Righi <arighi at develer.com> wrote:
> > 
> > > The page_cgroup infrastructure, currently available only for the memory
> > > cgroup controller, can be used to store the owner of each page and
> > > opportunely track the writeback IO. This information is encoded in
> > > the upper 16-bits of the page_cgroup->flags.
> > > 
> > > A owner can be identified using a generic ID number and the following
> > > interfaces are provided to store a retrieve this information:
> > > 
> > >   unsigned long page_cgroup_get_owner(struct page *page);
> > >   int page_cgroup_set_owner(struct page *page, unsigned long id);
> > >   int page_cgroup_copy_owner(struct page *npage, struct page *opage);
> > 
> > My immediate observation is that you're not really tracking the "owner"
> > here - you're tracking an opaque 16-bit token known only to the block
> > controller in a field which - if changed by anybody other than the block
> > controller - will lead to mayhem in the block controller.  I think it
> > might be clearer - and safer - to say "blkcg" or some such instead of
> > "owner" here.
> > 
> 
> Basically the idea here was to be as generic as possible and make this
> feature potentially available also to other subsystems, so that cgroup
> subsystems may represent whatever they want with the 16-bit token.
> However, no more than a single subsystem may be able to use this feature
> at the same time.
> 
> > I'm tempted to say it might be better to just add a pointer to your
> > throtl_grp structure into struct page_cgroup.  Or maybe replace the
> > mem_cgroup pointer with a single pointer to struct css_set.  Both of
> > those ideas, though, probably just add unwanted extra overhead now to gain
> > generality which may or may not be wanted in the future.
> 
> The pointer to css_set sounds good, but it would add additional space to
> the page_cgroup struct. Now, page_cgroup is 40 bytes (in 64-bit arch)
> and all of them are allocated at boot time. Using unused bits in
> page_cgroup->flags is a choice with no overhead from this point of view.

I think John suggested replacing mem_cgroup pointer with css_set so that
size of the strcuture does not increase but it leads extra level of 
indirection.

Thanks
Vivek
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