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    SubjectRe: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock
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    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);

    __mem_cgroup_update_page_stat(pc, idx, charge);
    unlock_page_cgroup(pc);
    }

    __mem_cgroup_update_page_stat() has a switch statement that updates all of the
    MEMCG_NR_FILE_{MAPPED,DIRTY,WRITEBACK,WRITEBACK_TEMP,UNSTABLE_NFS} counters
    using the following form:
    switch (idx) {
    case MEMCG_NR_FILE_MAPPED:
    if (charge) {
    if (!PageCgroupFileMapped(pc))
    SetPageCgroupFileMapped(pc);
    else
    val = 0;
    } else {
    if (PageCgroupFileMapped(pc))
    ClearPageCgroupFileMapped(pc);
    else
    val = 0;
    }
    idx = MEM_CGROUP_STAT_FILE_MAPPED;
    break;

    ...
    }

    /*
    * Preemption is already disabled. We can use __this_cpu_xxx
    */
    if (val > 0) {
    __this_cpu_inc(mem->stat->count[idx]);
    } else if (val < 0) {
    __this_cpu_dec(mem->stat->count[idx]);
    }

    In my current tree, irq is never saved/restored by cgroup locking code. To
    protect against interrupt reentrancy, trylock_page_cgroup() is used. As the
    comment indicates, this makes the new counters fuzzy.

    --
    Greg
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