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    SubjectRe: [PATCH v3 2/2] Make hard lockup detection use timestamps
    On Fri, Jul 29, 2011 at 04:12:32PM -0700, ZAK Magnus wrote:
    > Are you saying that any call to printk() will touch the watchdogs? I
    > wasn't aware of that and it doesn't seem to comply with my
    > observations too well, either. Then again, at the moment I don't
    > understand some of the things I'm currently seeing so I could just be
    > wrong.

    I believe the serial console write is the source of all the
    touch_nmi_watchdogs that result from a printk.

    drivers/tty/serial/8250.c::serial8250_console_write()

    Cheers,
    Don

    >
    > On Fri, Jul 29, 2011 at 1:55 PM, Don Zickus <dzickus@redhat.com> wrote:
    > > On Thu, Jul 28, 2011 at 05:16:00PM -0700, ZAK Magnus wrote:
    > >> No news?
    > >>
    > >> I've been testing and looking into issues and I realized dump_stack()
    > >> calls touch_nmi_watchdog(). That wrecks what the patch is trying to do
    > >> so I'm changing it to save the trace and print it later after the
    > >> stall has completed. This would also resolve some other things you
    > >> were saying weren't so good. Hopefully the logic is similar enough
    > >> that some things you may have learned still apply.
    > >
    > > So yeah, the acting of printing was resesting the softlockup counter and
    > > delaying it forever.  In parallel, rcu has its own stall detector that was
    > > going off after a minute or two.
    > >
    > > Once I routed the printk to trace_printk and disabled dump_stack,
    > > everything started working as expected.
    > >
    > > Now the question is how to avoid shooting ourselves in the foot by
    > > printk'ing a message without resetting the hard/soft lock watchdogs.
    > >
    > > I'll have to think about how to do that.  If you can come up with any
    > > ideas let me know.
    > >
    > > We almost need a quiet dump_stack that dumps to a buffer instead of the
    > > console.  But I am not sure that is worth the effort.
    > >
    > > Hmm.
    > >
    > > Cheers,
    > > Don
    > >
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