Messages in this thread | | | Date | Wed, 21 Jan 2009 19:32:48 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [RFC][PATCH 4/4] cgroup-memcg fix frequent EBUSY at rmdir v2 |
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On Wed, 21 Jan 2009 02:00:56 -0800 Paul Menage <menage@google.com> wrote:
> On Tue, Jan 20, 2009 at 2:47 AM, KAMEZAWA Hiroyuki > <kamezawa.hiroyu@jp.fujitsu.com> wrote: > > CGRP_NOTIFY_ON_RELEASE, > > + /* Someone calls rmdir() and is wating for this cgroup is released */ > > /* A thread is in rmdir() waiting to destroy this cgroup */ > > Also document that it can only be set/cleared when you're holding the > inode_sem for the cgroup directory. And we should probably move this > enum inside cgroup.c, since nothing in the header file uses it. > > > + CGRP_WAIT_ON_RMDIR, > > };
Hmm, ok. move this all enum to cgroup.c ?
> > > > > struct cgroup { > > @@ -350,7 +352,7 @@ int cgroup_is_descendant(const struct cg > > struct cgroup_subsys { > > struct cgroup_subsys_state *(*create)(struct cgroup_subsys *ss, > > struct cgroup *cgrp); > > - void (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp); > > + int (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp); > > Can you update the documentation to indicate what an error result from > pre_destroy indicates? Can pre_destroy() be called multiple times for > the same subsystem/cgroup? >
yes, after this, memcg will return -EBUSY in some special cases. (patches are on my stack.) We'll have -EBUSY situation especially on swap-less system.
> > + > > + /* wake up rmdir() waiter....it should fail.*/ > > /* Wake up rmdir() waiter - the rmdir should fail since the cgroup is > no longer empty */ > > But is this safe? If we do a pre-destroy, is it OK to let new tasks > into the cgroup? > Current memcg allows it. (so, I removed "obsolete" flag in memcg and asked you to add css_tryget().)
> > @@ -2446,6 +2461,8 @@ static long cgroup_create(struct cgroup > > > > mutex_unlock(&cgroup_mutex); > > mutex_unlock(&cgrp->dentry->d_inode->i_mutex); > > + if (wakeup_on_rmdir(parent)) > > + cgroup_rmdir_wakeup_waiters(); > > I don't think that there can be a waiter, since rmdir() would hold the > parent's inode semaphore, which would block this thread before it gets > to cgroup_create() > Oh, I see. I missed that. I'll remove this.
> > +DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq); > > + > > +static void cgroup_rmdir_wakeup_waiters(void) > > +{ > > + wake_up_all(&cgroup_rmdir_waitq); > > +} > > + > > I think you can merge wakeup_on_rmdir() and > cgroup_rmdir_wakeup_waiters() into a single function, > cgroup_wakeup_rmdir(struct cgroup *) > will try.
> > > > > + if (signal_pending(current)) > > + return -EINTR; > > I think it would be better to move this check to after we've already > failed on cgroup_clear_css_refs(). That way we can't fail with an > EINTR just because we raced with a signal on the way into rmdir() - we > have to actually hit the EBUSY and try to sleep.
Ok, will move.
> > + ret = cgroup_call_pre_destroy(cgrp); > > + if (ret == -EBUSY) > > + return -EBUSY; > > What about other potential error codes? If the subsystem's only > allowed to return 0 or EBUSY, then we should check for that. >
Hmm, subsystem may return -EPERM or some.. I'll change this to
if (!ret) return ret;
Thank you for review. very helpful. I'll consider more.
-Kame
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