Re: Context switch latency in tickless isolated CPU

[Mark Hounschell]

On 08/22/2016 11:37 AM, Paul E. McKenney wrote:
> 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

First paragraph scenario is the one I was referring.

Thanks
Mark

> > 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