Re: [PATCH] nohz: prevent tilegx network driver interrupts

On 07/20/2015 05:49 PM, Frederic Weisbecker wrote:

On Mon, Jul 20, 2015 at 05:22:12PM -0400, Chris Metcalf wrote:

quoted

On 07/11/2015 10:30 AM, Frederic Weisbecker wrote:

quoted

On Fri, Jul 10, 2015 at 03:05:02PM -0400, Chris Metcalf wrote:

quoted

The tilegx chips typically don't do cpu offlining anyway, since
we've never really found a usecase, so whatever you boot with
you always have available.  We do have support for a bare-metal
mode which you can run on some of the cores, so you may start
with fewer than cpu_possible actually running, but it will always
be that same set of cores.

And that bare metal mode runs out of Linux?

The bare metal environment runs on cpus that have been marked
as unavailable to Linux, so Linux just sees them as permanently
offlined.  There is a BME driver (which we haven't upstreamed,
since the BME isn't upstreamed either) that arranges to share
memory between the BME and Linux.

I don't think that many customers are using the BME in any
case.  We push all of them towards using our dataplane mode
instead, since it almost always works just as well from a
performance perspective, and is easier to develop code for.

So bare metal mode is different than dataplane mode, right?
Where bare metal mode offlines the CPU and IIUC dataplane mode
instead uses CPUs that are available to Linux, just isolated
with nohz and various affinity stuff, right?

Yes, exactly.  Cores running the bare metal environment are
NOT running Linux at all, just talking directly to the Tilera
hypervisor.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com