Re: [PATCH][RESEND]RE: [Xen-devel] [PATCH] Fix softlockup issue after vcpu hotplug

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 2/2/07 01:39, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx> wrote:

> Odder still, the softlockup threads are SCHED_FIFO/99 in 2.6.16 too. So the
> main change is that rather than an explicit sleep of one second, the thread
> now sleeps as TASK_INTERRUPTIBLE. I wonder how it gets kicked back to
> TASK_RUNNING?
> 
> http://lwn.net/Articles/173648/ is worrying since it seems to state that the
> patch intends to make the thread timer-interrupt driven rather than softirq
> timer driven. If that means it is jiffy-ticker driver, then perhaps the
> softlockup module is incompatible with tickless idle mode (no-idle-hz).

Okay, I now see how this works -- the thread is kicked from
softlockup_tick(), from the timer ISR. So this wakeup event is hidden from
next_timer_interrupt(), which only searches timer wheels and hrtimers.

The strictly correct fix here is to make next_timer_interrupt()
softlockup-aware. I would say it is currently incorrect in the presence of
softlockup since it is not doing its job (telling an idle process what the
next time-based event is that it must wake up for).

We can do this by adding a softlockup_get_next_event(), called from the
bottom of next_timer_interrupt(). I would pass it the current return value
and have it return an adjusted value: so in the absence of softlockup it
would simply return its argument unmodified. In the presence of softlockup
it would return a sooner value if softlockup is the next event to fire.

Do you want to try coding this up?

 -- Keir



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