Re: Xen on ARM IRQ latency and scheduler overhead

From: Dario Faggioli <dario.faggioli@citrix.com>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: george.dunlap@eu.citrix.com, edgar.iglesias@xilinx.com,
	julien.grall@arm.com, xen-devel@lists.xen.org
Subject: Re: Xen on ARM IRQ latency and scheduler overhead
Date: Fri, 17 Feb 2017 12:02:41 +0100	[thread overview]
Message-ID: <1487329361.6732.76.camel@citrix.com> (raw)
In-Reply-To: <alpine.DEB.2.10.1702161439490.9566@sstabellini-ThinkPad-X260>

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Just very quickly...

On Thu, 2017-02-16 at 15:07 -0800, Stefano Stabellini wrote:
> (XEN) Active queues: 1
> (XEN)   default-weight     = 256
> (XEN) Runqueue 0:
> (XEN)   ncpus              = 4
> (XEN)   cpus               = 0-3
> (XEN)   max_weight         = 256
> (XEN)   instload           = 1
> (XEN)   aveload            = 3208 (~1%)
> (XEN) l(XEN)    idlers: 00000000,00000000,00000000,0000000a
> a(XEN)  tickled: 00000000,00000000,00000000,00000000
> t(XEN)  fully idle cores: 00000000,00000000,00000000,0000000a
> e(XEN) Domain info:
> n(XEN)  Domain: 0 w 256 v 4
> c(XEN)    1: [0.0] flags=2 cpu=0 credit=10500000 [w=256] load=3170 (~1%)
> (XEN)     2: y[0.1] flags=0 cpu=1  credit=10500000 [w=256]( load=131072 (~50%)
> (XEN)     3: n[0.2] flags=0 cpu=2s credit=10500000 [w=256]) load=131072 (~50%):
> (XEN)     4:  [0.3] flags=0 cpu=3m credit=10500000 [w=256]a load=131072 (~50%)x
>
Status of vcpus 2, 3 and 4 is a bit weird. I'll think about it.

> =(XEN)  Domain: 1 w 256 v 1
> 1(XEN)    5: 1[1.0] flags=2 cpu=2 credit=9713074 [w=256] load=56 (~0%)
> (XEN) Runqueue info:
> 6(XEN) runqueue 0:
> 9(XEN) CPUs info:
> 0(XEN) CPU[00]   runq=0, sibling=00000000,00000000,00000000,00000001, wcore=00000000,00000000,00000000,00000001
>
This tells me that nr_cpu_ids is very big (I think it tells it is 128,
i.e., ARM default), which means cpumask_t-s are huge.

What does `xl info' says. On my (x86) test box, it's like this:

 ...
 nr_cpus                : 16
 max_cpu_id             : 63
 ...

(and I have NR_CPUS=256, i.e., x86 the default).

Cpumasks being bigger also means cpumask operation being slower, and
this matters quite a bit in Credit2, because we use cpumasks a lot (but
also in Credit1, because we use cpumasks a little less than in Credit2,
but still quite a bit).

Isn't there a way, on ARM, to figure out online that you're not going
to have 128 cpus in the platform?

Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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