Re: Context switch latency in tickless isolated CPU

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

On Mon, Aug 22, 2016 at 11:12:45AM -0400, Mark Hounschell wrote:

quoted

On 08/22/2016 10:48 AM, Paul E. McKenney wrote:

quoted

On Mon, Aug 22, 2016 at 05:40:03PM +0800, GeHao Kang wrote:

quoted

On Sun, Aug 21, 2016 at 10:53 PM, Paul E. McKenney
[off-list ref] wrote:

quoted

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

quoted

Mark

quoted

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