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    SubjectRe: [BUGFIX][PATCH -mmotm] memcg: avoid oom-killing innocent task in case of use_hierarchy
    * nishimura@mxp.nes.nec.co.jp <nishimura@mxp.nes.nec.co.jp> [2009-11-25 08:49:10]:

    > On Tue, 24 Nov 2009 22:34:02 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    > > * Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp> [2009-11-24 23:00:29]:
    > >
    > > > On Tue, 24 Nov 2009 19:01:54 +0530
    > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    > > >
    > > > > On Tue, Nov 24, 2009 at 11:27 AM, Daisuke Nishimura
    > > > > <nishimura@mxp.nes.nec.co.jp> wrote:
    > > > > > task_in_mem_cgroup(), which is called by select_bad_process() to check whether
    > > > > > a task can be a candidate for being oom-killed from memcg's limit, checks
    > > > > > "curr->use_hierarchy"("curr" is the mem_cgroup the task belongs to).
    > > > > >
    > > > > > But this check return true(it's false positive) when:
    > > > > >
    > > > > >        <some path>/00          use_hierarchy == 0      <- hitting limit
    > > > > >          <some path>/00/aa     use_hierarchy == 1      <- "curr"
    > > > > >
    > > > > > This leads to killing an innocent task in 00/aa. This patch is a fix for this
    > > > > > bug. And this patch also fixes the arg for mem_cgroup_print_oom_info(). We
    > > > > > should print information of mem_cgroup which the task being killed, not current,
    > > > > > belongs to.
    > > > > >
    > > > >
    > > > > Quick Question: What happens if <some path>/00 has no tasks in it
    > > > > after your patches?
    > > > >
    > > > Nothing would happen because <some path>/00 never hit its limit.
    > >
    > > Why not? I am talking of a scenario where <some path>/00 is set to a
    > > limit (similar to your example) and hits its limit, but the groups
    > > under it have no limits, but tasks. Shouldn't we be scanning
    > > <some path>/00/aa as well?
    > >
    > > >
    > > > The bug that this patch fixes is:
    > > >
    > > > - create a dir <some path>/00 and set some limits.
    > > > - create a sub dir <some path>/00/aa w/o any limits, and enable hierarchy.
    > > > - run some programs in both in 00 and 00/aa. programs in 00 should be
    > > > big enough to cause oom by its limit.
    > > > - when oom happens by 00's limit, tasks in 00/aa can also be killed.
    > > >
    > >
    > > To be honest, the last part is fair, specifically if 00/aa has a task
    > > that is really the heaviest task as per the oom logic. no? Are you
    > > suggesting that only tasks in <some path>/00 should be selected by the
    > > oom logic?
    > >
    > All of your comments would be rational if hierarchy is enabled in 00(it's
    > also enabled in 00/aa automatically in this case).
    > I'm saying about the case where it's disabled in 00 but enabled in 00/aa.
    >

    OK, I misunderstood the example then, so even though hierarchy is
    disabled, we kill a task in 00/aa when 00 hits the limit. Thanks for
    clarifying.

    > In this scenario, charges by tasks in 00/aa is(and should not be) charged to 00.
    > And oom caused by 00's limit should not affect the task in 00/aa.
    >
    >
    > Regards,
    > Daisuke Nishimura.
    >
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    >

    --
    Balbir
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