On Mon, Aug 29, 2011 at 12:57 AM, Johannes Weiner [off-list ref] wrote:

On Mon, Aug 29, 2011 at 12:22:02AM -0700, Ying Han wrote:

quoted

fix hierarchy_walk() to hold a reference to first mem_cgroup

The first mem_cgroup returned from hierarchy_walk() is used to
terminate a round-trip. However there is no reference hold on
that which the first could be removed during the walking. The
patch including the following change:

1. hold a reference on the first mem_cgroup during the walk.
2. rename the variable "root" to "target", which we found using
"root" is confusing in this content with root_mem_cgroup. better
naming is welcomed.

Thanks for the report.

This was actually not the only case that could lead to overlong (not
necessarily endless) looping.

With several scanning threads, a single thread may not encounter its
first cgroup again for a long time, as the other threads would visit
it.

Yes, that makes sense. And I think i found a issue on my patch which
it leaks reference count on the mem (first) which I can not do "rmdir"
after some memory pressure tests. So, please ignore the patch for now.

I changed this to use scan generations.  Restarting the scan from id 0
starts the next scan generation.  The iteration function returns NULL
if the generation changed since a loop was started.

This way, iterators can reliably detect whether they should call it
quits without any requirements for previously encountered memcgs.

Ok, so if I have multiple threads hitting pressure under the same zone
and same memcg hierarchy tree, they all contribute to the single
iteration loop. And all the reclaimers will terminate if they together
made a full iteration under the hierarchy?

If so, i will look at your patch and no need for the fix i posted
early on. Meanwhile, I would be interested to look at some performance
data since the later one should save some cpu cycles going through
more memcgs than after the change.

Thanks

--Ying

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>