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    SubjectRe: [PATCH 1/2] memcg: cgroup fix rmdir hang
    On Tue, 23 Jun 2009 16:08:54 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >
    > Now, cgroup has a logic to wait until ready-to-rmdir for avoiding
    > frequent -EBUSY at rmdir.
    > (See Commit ec64f51545fffbc4cb968f0cea56341a4b07e85a
    > cgroup: fix frequent -EBUSY at rmdir.
    >
    > Nishimura-san reported bad case for waiting and This is a fix to
    > make it reliable. A thread waiting for thread cannot be waken up
    > when a refcnt gotten by css_tryget() isn't put immediately.
    > (Original code assumed css_put() will be called soon.)
    >
    > memcg has this case and this is a fix for the problem. This adds
    > retry_rmdir() callback to subsys and check we can sleep or not.
    >
    > Note: another solution will be adding "rmdir state" to subsys.
    > But it will be much complicated than this do-enough-check solution.
    >
    > Changelog v1 -> v2:
    > - splitted into 2 patches. This just includes retry_rmdir() modification.
    >
    > Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

    Looks good to me.

    Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

    I've been testing with [2/2] applied (because both of these patches are
    necessary to fix this problem completely), and it works fine so far.
    But I want to test in more test cases.


    Thanks,
    Daisuke Nishimura.

    > ---
    > Documentation/cgroups/cgroups.txt | 11 +++++++++++
    > include/linux/cgroup.h | 1 +
    > kernel/cgroup.c | 20 +++++++++++++++++++-
    > mm/memcontrol.c | 14 ++++++++++++--
    > 4 files changed, 43 insertions(+), 3 deletions(-)
    >
    > Index: fix-rmdir-cgroup/include/linux/cgroup.h
    > ===================================================================
    > --- fix-rmdir-cgroup.orig/include/linux/cgroup.h
    > +++ fix-rmdir-cgroup/include/linux/cgroup.h
    > @@ -374,6 +374,7 @@ struct cgroup_subsys {
    > struct cgroup_subsys_state *(*create)(struct cgroup_subsys *ss,
    > struct cgroup *cgrp);
    > int (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
    > + int (*retry_rmdir)(struct cgroup_subsys *ss, struct cgroup *cgrp);
    > void (*destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
    > int (*can_attach)(struct cgroup_subsys *ss,
    > struct cgroup *cgrp, struct task_struct *tsk);
    > Index: fix-rmdir-cgroup/kernel/cgroup.c
    > ===================================================================
    > --- fix-rmdir-cgroup.orig/kernel/cgroup.c
    > +++ fix-rmdir-cgroup/kernel/cgroup.c
    > @@ -636,6 +636,23 @@ static int cgroup_call_pre_destroy(struc
    > }
    > return ret;
    > }
    > +/*
    > + * Call subsys's retry_rmdir() handler. If this returns non-Zero, we retry
    > + * rmdir immediately and call pre_destroy again.
    > + */
    > +static int cgroup_check_retry_rmdir(struct cgroup *cgrp)
    > +{
    > + struct cgroup_subsys *ss;
    > + int ret = 0;
    > +
    > + for_each_subsys(cgrp->root, ss)
    > + if (ss->pre_destroy) {
    > + ret = ss->retry_rmdir(ss, cgrp);
    > + if (ret)
    > + break;
    > + }
    > + return ret;
    > +}
    >
    > static void free_cgroup_rcu(struct rcu_head *obj)
    > {
    > @@ -2722,7 +2739,8 @@ again:
    >
    > if (!cgroup_clear_css_refs(cgrp)) {
    > mutex_unlock(&cgroup_mutex);
    > - schedule();
    > + if (!cgroup_check_retry_rmdir(cgrp))
    > + schedule();
    > finish_wait(&cgroup_rmdir_waitq, &wait);
    > clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
    > if (signal_pending(current))
    > Index: fix-rmdir-cgroup/mm/memcontrol.c
    > ===================================================================
    > --- fix-rmdir-cgroup.orig/mm/memcontrol.c
    > +++ fix-rmdir-cgroup/mm/memcontrol.c
    > @@ -1457,8 +1457,6 @@ __mem_cgroup_commit_charge_swapin(struct
    > }
    > rcu_read_unlock();
    > }
    > - /* add this page(page_cgroup) to the LRU we want. */
    > -
    > }
    >
    > void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
    > @@ -2571,6 +2569,17 @@ static int mem_cgroup_pre_destroy(struct
    > return mem_cgroup_force_empty(mem, false);
    > }
    >
    > +static int mem_cgroup_retry_rmdir(struct cgroup_subsys *ss,
    > + struct cgroup *cont)
    > +{
    > + struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
    > +
    > + if (res_counter_read_u64(&mem->res, RES_USAGE))
    > + return 1;
    > + return 0;
    > +}
    > +
    > +
    > static void mem_cgroup_destroy(struct cgroup_subsys *ss,
    > struct cgroup *cont)
    > {
    > @@ -2610,6 +2619,7 @@ struct cgroup_subsys mem_cgroup_subsys =
    > .subsys_id = mem_cgroup_subsys_id,
    > .create = mem_cgroup_create,
    > .pre_destroy = mem_cgroup_pre_destroy,
    > + .retry_rmdir = mem_cgroup_retry_rmdir,
    > .destroy = mem_cgroup_destroy,
    > .populate = mem_cgroup_populate,
    > .attach = mem_cgroup_move_task,
    > Index: fix-rmdir-cgroup/Documentation/cgroups/cgroups.txt
    > ===================================================================
    > --- fix-rmdir-cgroup.orig/Documentation/cgroups/cgroups.txt
    > +++ fix-rmdir-cgroup/Documentation/cgroups/cgroups.txt
    > @@ -500,6 +500,17 @@ there are not tasks in the cgroup. If pr
    > rmdir() will fail with it. From this behavior, pre_destroy() can be
    > called multiple times against a cgroup.
    >
    > +int retry_rmdir(struct cgroup_subsys *ss, struct cgroup *cgrp);
    > +
    > +Called at rmdir right after the kernel finds there are remaining refcnt on
    > +subsystems after pre_destroy(). When retry_rmdir() returns 0, the caller enter
    > +sleep and wakes up when css's refcnt goes down to 0 by css_put().
    > +When this returns 1, the caller doesn't sleep and retry rmdir immediately.
    > +This is useful when the subsys knows remaining css's refcnt is not temporal
    > +and to calling pre_destroy() again is proper way to remove that.
    > +(or proper way to retrun -EBUSY.)
    > +
    > +
    > int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
    > struct task_struct *task)
    > (cgroup_mutex held by caller)
    >
    > --
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    > see: http://www.linux-mm.org/ .
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