Re: [PATCH] tick: Detect and fix jiffies update stall

[Paul E. McKenney]

On Wed, Feb 02, 2022 at 03:19:51AM +0100, Frederic Weisbecker wrote:
> On Tue, Feb 01, 2022 at 05:49:34PM -0800, Paul E. McKenney wrote:
> > On Wed, Feb 02, 2022 at 01:01:07AM +0100, Frederic Weisbecker wrote:
> > > On some rare cases, the timekeeper CPU may be delaying its jiffies
> > > update duty for a while. Known causes include:
> > > 
> > > * The timekeeper is waiting on stop_machine in a MULTI_STOP_DISABLE_IRQ
> > >   or MULTI_STOP_RUN state. Disabled interrupts prevent from timekeeping
> > >   updates while waiting for the target CPU to complete its
> > >   stop_machine() callback.
> > > 
> > > * The timekeeper vcpu has VMEXIT'ed for a long while due to some overload
> > >   on the host.
> > > 
> > > Detect and fix these situations with emergency timekeeping catchups.
> > > 
> > > Original-patch-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > 
> > Nice, thank you!
> > 
> > So I should revert your earlier patch, apply this one, and then test
> > the result?
> 
> No need to revert the nohz_full fix, this new one deals with non-dynticks
> issues. This way we cover every timekeeper stall situations:
> 
> _ dynticks-idle is handled on IRQ entry
> 
> _ full dynticks is handled on IRQ entry in case of CPU 0 (traditional nohz_full
>   timekeeper) timekeeping stall. Let's hope we won't need to handle syscalls and
>   faults as well but we'll see...
>   
> _ periodic ticks are now handled on the tick.
> 
> So you just need to apply this patch on your dev branch for testing.

I have pulled it in, thank you!  I will beat on it.

I am guessing that this goes up some other path to mainline, so I have
marked it "EXP".

							Thanx, Paul