Re: [RFC] rework mem_cgroup iterator

[Glauber Costa]

On 11/13/2012 04:30 PM, Michal Hocko wrote:
> Hi all,
> this patch set tries to make mem_cgroup_iter saner in the way how it
> walks hierarchies. css->id based traversal is far from being ideal as it
> is not deterministic because it depends on the creation ordering.
> 
> Diffstat looks promising but it is fair the say that the biggest cleanup is
> just css_get_next removal. The memcg code has grown a bit but I think it is
> worth the resulting outcome (the sanity ;)).
> 
> The first patch fixes a potential misbehaving which I haven't seen but the
> fix is needed for the later patches anyway. We could take it alone as well
> but I do not have any bug report to base the fix on.
> 
> The second patch replaces css_get_next by cgroup iterators which are
> scheduled for 3.8 in Tejun's tree and I depend on the following two patches:
> fe1e904c cgroup: implement generic child / descendant walk macros
> 7e187c6c cgroup: use rculist ops for cgroup->children
> 
> The third patch is an attempt for simplification of the mem_cgroup_iter. It
> basically removes all css usages to make the code easier. The next patch
> removes the big while(!memcg) loop around the iterating logic. It could have
> been folded into #3 but I rather have the rework separate from the code
> moving noise.
> 
> The last patch just removes css_get_next as there is no user for it any
> longer.
> 
> I am also thinking that leaf-to-root iteration makes more sense but this
> patch is not included in the series yet because I have to think some
> more about the justification.
> 
> So far I didn't get to testing but I am posting this early if everybody is
> OK with this change.
> 
> Any thoughts?
> 

Why can't we reuse the scheduler iterator and move it to kernel/cgroup.c
? It already exists, provide sane ordering, and only relies on parent
information - which cgroup core already have - to do the walk.

The only minor problem is that we'll have to handle the damn
use_hierarchy case, so we may not be able to blindly rely on
cgroup->parent. But maybe we can, if we don't guarantee any particular
leaf-order.

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