Messages in this thread | | | Date | Wed, 30 Jan 2013 16:08:27 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] printk: Avoid softlockups in console_unlock() |
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On Tue, 29 Jan 2013 15:54:24 +0100 Jan Kara <jack@suse.cz> wrote:
> > So I was testing the attached patch which does what we discussed. The bad > > news is I was able to trigger a situation (twice) when suddently sda > > disappeared and thus all IO requests failed with EIO. There is no trace of > > what's happened in the kernel log. I'm guessing that disabled interrupts on > > the printing CPU caused scsi layer to time out for some request and fail the > > device. So where do we go from here? > Andrew? I guess this fell off your radar via the "hrm, strange, need to > have a closer look later" path?
urgh. I was hoping that if we left it long enough, one of both of us would die :(
I fear we will rue the day when we changed printk() to bounce some of its work up to a kernel thread.
> Currently I'd be inclined to return to my original solution...
Can we make it smarter? Say, take a peek at the current softlockup/nmi-watchdog intervals, work out how for how long we can afford to keep interrupts disabled and then use that period and sched_clock() to work out if we're getting into trouble? IOW, remove the hard-wired "1000" thing which will always be too high or too low for all situations.
Implementation-wise, that would probably end up adding a kernel-wide function along the lines of
/* * Return the maximum number of nanosecond for which interrupts may be disabled * on the current CPU */ u64 max_interrupt_disabled_duration(void) { return min(sortirq duration, nmi watchdog duration); }
Thinking ahead...
Other kernel sites which know they can disable interrupts for a long time can perhaps use this.
Later, realtimeish systems (for example machine controllers) might want to add a kernel tunable so they can set the max_interrupt_disabled_duration() return value much lower.
To make that more accurate, we could add per-cpu, per-irq variables to record sched_clock() when each CPU enters the interrupt, so the comment becomes
/* * Return the remaining maximum number of nanosecond for which interrupts may * be disabled on the current CPU */
This may all be crazy and hopefully we'll never do it, but the design should permit such things from day one if practical.
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