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    SubjectRe: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock
    On Tue, 13 Apr 2010 23:55:12 -0700
    Greg Thelen <gthelen@google.com> wrote:

    > On Thu, Mar 18, 2010 at 8:00 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > On Fri, 19 Mar 2010 08:10:39 +0530
    > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    > >
    > >> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-03-19 10:23:32]:
    > >>
    > >> > On Thu, 18 Mar 2010 21:58:55 +0530
    > >> > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    > >> >
    > >> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-03-18 13:35:27]:
    > >> >
    > >> > > > Then, no probelm. It's ok to add mem_cgroup_udpate_stat() indpendent from
    > >> > > > mem_cgroup_update_file_mapped(). The look may be messy but it's not your
    > >> > > > fault. But please write "why add new function" to patch description.
    > >> > > >
    > >> > > > I'm sorry for wasting your time.
    > >> > >
    > >> > > Do we need to go down this route? We could check the stat and do the
    > >> > > correct thing. In case of FILE_MAPPED, always grab page_cgroup_lock
    > >> > > and for others potentially look at trylock. It is OK for different
    > >> > > stats to be protected via different locks.
    > >> > >
    > >> >
    > >> > I _don't_ want to see a mixture of spinlock and trylock in a function.
    > >> >
    > >>
    > >> A well documented well written function can help. The other thing is to
    > >> of-course solve this correctly by introducing different locking around
    > >> the statistics. Are you suggesting the later?
    > >>
    > >
    > > No. As I wrote.
    > >        - don't modify codes around FILE_MAPPED in this series.
    > >        - add a new functions for new statistics
    > > Then,
    > >        - think about clean up later, after we confirm all things work as expected.
    >
    > I have ported Andrea Righi's memcg dirty page accounting patches to latest
    > mmtom-2010-04-05-16-09. In doing so I have to address this locking issue. Does
    > the following look good? I will (of course) submit the entire patch for review,
    > but I wanted make sure I was aiming in the right direction.
    >
    > void mem_cgroup_update_page_stat(struct page *page,
    > enum mem_cgroup_write_page_stat_item idx, bool charge)
    > {
    > static int seq;
    > struct page_cgroup *pc;
    >
    > if (mem_cgroup_disabled())
    > return;
    > pc = lookup_page_cgroup(page);
    > if (!pc || mem_cgroup_is_root(pc->mem_cgroup))
    > return;
    >
    > /*
    > * This routine does not disable irq when updating stats. So it is
    > * possible that a stat update from within interrupt routine, could
    > * deadlock. Use trylock_page_cgroup() to avoid such deadlock. This
    > * makes the memcg counters fuzzy. More complicated, or lower
    > * performing locking solutions avoid this fuzziness, but are not
    > * currently needed.
    > */
    > if (irqs_disabled()) {
    > if (! trylock_page_cgroup(pc))
    > return;
    > } else
    > lock_page_cgroup(pc);
    >

    I prefer trylock_page_cgroup() always.

    I have another idea fixing this up _later_. (But I want to start from simple one.)

    My rough idea is following. Similar to your idea which you gave me before.

    ==
    DEFINE_PERCPU(account_move_ongoing);
    DEFINE_MUTEX(move_account_mutex):

    void memcg_start_account_move(void)
    {
    mutex_lock(&move_account_mutex);
    for_each_online_cpu(cpu)
    per_cpu(cpu, account_move_ongoing) += 1;
    mutex_unlock(&move_account_mutex);
    /* Wait until there are no lockless update */
    synchronize_rcu();
    return;
    }

    void memcg_end_account_move(void)
    {
    mutex_lock(&move_account_mutex);
    for_each_online_cpu(cpu)
    per_cpu(cpu, account_move_ongoing) -= 1;
    mutex_unlock(&move_account_mutex);
    }

    /* return 1 when we took lock, return 0 if lockess OPs is guarantedd to be safe */
    int memcg_start_filecache_accounting(struct page_cgroup *pc)
    {
    rcu_read_lock();
    smp_rmb();
    if (!this_cpu_read(move_account_ongoing))
    return 0; /* no move account is ongoing */
    lock_page_cgroup(pc);
    return 1;
    }

    void memcg_end_filecache_accounting(struct page_cgroup *pc, int unlock)
    {
    if (unlock)
    unlock_page_cgroup(pc);

    rcu_read_unlock();
    }

    and call memcg_start_account_move()/end_account_move() in the start/end of
    migrainting chunk of pages.

    Bye.
    -Kame




















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