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    SubjectRe: [RFC -v2] panic_on_oom_timeout
    On Wed 17-06-15 22:59:54, Tetsuo Handa wrote:
    > Michal Hocko wrote:
    [...]
    > > But you have a point that we could have
    > > - constrained OOM which elevates oom_victims
    > > - global OOM killer strikes but wouldn't start the timer
    > >
    > > This is certainly possible and timer_pending(&panic_on_oom) replacing
    > > oom_victims check should help here. I will think about this some more.
    >
    > Yes, please.

    Fixed in my local version. I will post the new version of the patch
    after we settle with the approach.

    > > The important thing is to decide what is the reasonable way forward. We
    > > have two two implementations of panic based timeout. So we should decide
    > > - Should we add a panic timeout at all?
    > > - Should be the timeout bound to panic_on_oom?
    > > - Should we care about constrained OOM contexts?
    > > - If yes should they use the same timeout?
    > > - If no should each memcg be able to define its own timeout?
    > >
    > Exactly.
    >
    > > My thinking is that it should be bound to panic_on_oom=1 only until we
    > > hear from somebody actually asking for a constrained oom and even then
    > > do not allow for too large configuration space (e.g. no per-memcg
    > > timeout) or have separate mempolicy vs. memcg timeouts.
    >
    > My implementation comes from providing debugging hints when analyzing
    > vmcore of a stalled system. I'm posting logs of stalls after global OOM
    > killer struck because it is easy to reproduce. But what I have problem
    > is when a system stalled before the OOM killer strikes (I saw many cases
    > for customer's enterprise servers), for we don't have hints for guessing
    > whether memory allocator is the cause or not. Thus, my version tried to
    > emit warning messages using sysctl_memalloc_task_warn_secs .

    I can understand your frustration but I believe that a debugability is
    a separate topic and we should start by defining a reasonable _policy_
    so that an administrator has a way to handle potential OOM stalls
    reasonably and with a well defined semantic.

    > Ability to take care of constrained OOM contexts is a side effect of use of
    > per a "struct task_struct" variable. Even if we come to a conclusion that
    > we should not add a timeout for panic, I hope that a timeout for warning
    > about memory allocation stalls is added.
    >
    > > Let's start simple and make things more complicated later!
    >
    > I think we mismatch about what the timeout counters are for.

    --
    Michal Hocko
    SUSE Labs


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