Messages in this thread |  | | From | Paul Menage <> | Date | Wed, 13 Apr 2011 15:10:59 +0200 | Subject | Re: query: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach_task |
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On Thu, Apr 7, 2011 at 11:55 AM, Mike Galbraith <efault@gmx.de> wrote: > Greetings, > > Wrt these patches: > > https://lkml.org/lkml/2010/11/24/14 [PATCH 1/2] cgroup: Set CGRP_RELEASABLE when adding to a cgroup > https://lkml.org/lkml/2010/11/24/15 [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach_task > > I received a query regarding 2/2 because a large database company is > apparently moving tasks between cgroups frequently enough that their > database initialization time dropped from ~11 hours to ~4 hours when > they applied this patch.
That sounds like a problem in their user-space code too, although I agree that making cgroup moves faster is a good thing.
> > Curious why these got no traction. >
Apart from just my chronic lack of time to work on cgroups, there were a couple of issues:
1) we had trouble getting the semantics right for the release_agent notifications. Not that this is something that I suspect many people care about, but it has been part of the API since the cpuset days. I spent a while trying to juggle the way that release notifications were done (via an event counter rather than a simple flag) but never got them finished.
2) I have this nagging feeling that the synchronize_rcu() call in cgroup_attach_task() was protecting more than is obvious. Certainly when cgroups first went in, that synchronize_rcu() call meant that cgroup_rmdir() could guarantee that if the cgroup was empty, there were no threads in RCU-read sections accessing their old cgroup via their RCU-proected current->cgroups pointers, so objects could just be deleted at that point. A year or two ago we RCU-ified most/all of the cgroup deletion path, so this shouldn't be an issue now, but I'm still a bit worried that we missed something. I'm probably being over-paranoid though.
We're looking at testing these patches at Google, which will give a little more confidence.
There's a conflicting patchset (allowing moving entire processes by writing to cgroup.procs) that Ben Blum has been trying to get in for ages, and which has just gone in to -mm - the RCU change patches will likely need a bit of merge love.
Paul
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