Re: Context switch latency in tickless isolated CPU

[Paul E. McKenney]

On Mon, Aug 22, 2016 at 11:12:45AM -0400, Mark Hounschell wrote:
> On 08/22/2016 10:48 AM, Paul E. McKenney wrote:
> >On Mon, Aug 22, 2016 at 05:40:03PM +0800, GeHao Kang wrote:
> >>On Sun, Aug 21, 2016 at 10:53 PM, Paul E. McKenney
> >><paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >>>If latency is all you care about, one approach is to map the device
> >>>registers into userspace and do the I/O without assistance from the
> >>>kernel.
> >>In addition to the context switch latency, local interrupts are also
> >>closed during
> >>user_enter and user_exit of the context tracking. Therefore, the interrupt
> >>latency might be also increased on the isolated tickless CPU. That
> >>will degrade the
> >>real time performance. Are these two events determined?
> >
> >Hmmm...  Why would you be taking interrupts on your isolated tickless
> >CPUs?  Doesn't that defeat the purpose of designating them as isolated
> >and tickless?
> 
> Don't mean to butt in here but think about a "special" PCI card that
> does nothing but take an external interrupt or external interrupts
> from an outside source where the latency between the time it occurs
> on the outside and the time an isolated processor can act on that
> event. The IRQ of that card also being pinned/isolated to that
> processor. This is a very common thing in the RT world.

In this case, the host OS would see an event-driven real-time workload
from the PCI card, which would lead me to suggest -not- using NO_HZ_FULL
on the host OS.

Of course, if you are instead building an OS to run on the PCI card
itself, then the choice of configuration would depend on how the PCI
card was set up.  If it polled hardware, then NO_HZ_FULL on the PCI card
might work quite well.  But then you wouldn't have interrupts (on the
PCI card), so I am guessing that you mean the scenario covered in the
first paragraph.

Or am I missing your point?

							Thanx, Paul

> Mark
> 
> >The key point being that effective use of NO_HZ_FULL requires
> >careful configuration and complete understanding of your workload.
> >And it is quite possible that you instead need to use something
> >other than NO_HZ_FULL.
> >
> >If your question is instead "why must interrupts be disabled during
> >context tracking", I must defer to people who understand the x86
> >entry/exit code paths better than I do.
> >
> >							Thanx, Paul
> >
> >
>