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    SubjectRe: [feature] automatically detect hung TASK_UNINTERRUPTIBLE tasks
    On Mon, Dec 03, 2007 at 12:59:00PM +0100, Ingo Molnar wrote:
    > no. (that's why i added the '(or a kill -9)' qualification above - if
    > NFS is mounted noninterruptible then standard signals (such as Ctrl-C)
    > should not have an interrupting effect.)

    NFS is already interruptible with umount -f (I use that all the time...),
    but softlockup won't know that and throw the warning anyways.

    > your syslet snide comment aside (which is quite incomprehensible - a

    For the record I have no principle problem with syslets, just I do
    consider them roughly equivalent in end result to a explicit retry based
    AIO implementation.

    > retry based asynchonous IO model is clearly inferior even if it were
    > implemented everywhere), i do think that most if not all of these
    > supposedly "difficult to fix" codepaths are just on the backburner out
    > of lack of a clear blame vector.

    Hmm. -ENOPARSE. Can you please clarify?

    >
    > "audit thousands of callsites in 8 million lines of code first" is a
    > nice euphemism for hiding from the blame forever. We had 10 years for it

    Ok your approach is then to "let's warn about it and hope
    it will go away"

    > and it didnt happen. As we've seen it again and again, getting a
    > non-fatal reminder in the dmesg about the suckage is quite efficient at

    It's not universal suckage I would say, but sometimes unavoidable
    conditions. Now it is better of course to have these all TASK_KILLABLE,
    but then fixing that all in the kernel will probably a long term
    project. I'm not arguing against that, just forcing it through
    backtraces before even starting all that is probably not the right
    strategy to do that.

    > getting people to fix crappy solutions, and gives users and exact blame
    > point of where to start. That will create pressure to fix these
    > problems.

    After impacting the user base -- many of these conditions are infrequent
    enough that we will likely only see them during real production. Throwing
    warnings for lots of known cases is probably ok for a -mm kernel
    (where users expect things lik that), but not a "release" (be it
    Linus release or any kind of end user distribution) imho.

    I don't think there is a real alternative to code audit first
    (and someone doing all the work of fixing all these first)


    >
    > > > I think you are somehow confusing two issues: this patch in no way
    > > > declares that "long waits are bad" - if the user _choses_ to wait
    > > > for
    > >
    > > Throwing a backtrace is the kernel's way to declare something as bad.
    > > The only more clear ways to that I know of would be BUG or panic().
    >
    > there are various levels of declarig something bad, and you are quite
    > wrong to suggest that a BUG() would be the only recourse.

    I didn't write that, please reread my sentence..

    But we seem to agree that a backtrace is something "declared bad" anyways,
    which was my point.


    >
    > > > way to stop_ are quite likely bad".
    > >
    > > The user will just see the backtraces and think the kernel has
    > > crashed.
    >
    > i've just changed the message to:
    >
    > INFO: task keventd/5 blocked for more than 120 seconds.
    > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message

    That's better, but the backtrace is still there isn't it?

    Anyways I think I could live with it a one liner warning (if it's
    seriously rate limited etc.) and a sysctl to enable the backtraces;
    off by default. Or if you prefer that record
    the backtrace always in a buffer and make it available somewhere in /proc
    or /sys or /debug. Would that work for you?

    -Andi


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