linux-kernel - Re: [PATCH] x86/xen: Add "xen_timer

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Date:   Sat, 23 Mar 2019 11:41:51 +0100
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Dario Faggioli <dfaggioli@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>, thibodux@...il.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        oleksandr_andrushchenko@...m.com, tglx@...utronix.de,
        jgross@...e.com, ryan.thibodeaux@...rlab.io
Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option

Hi all,

On Sat, 23 Mar 2019 03:58:16 +0100
Dario Faggioli <dfaggioli@...e.com> wrote:

> On Fri, 2019-03-22 at 18:10 -0400, Boris Ostrovsky wrote:
> > On 3/22/19 2:29 PM, thibodux@...il.com wrote:  
> > > From: Ryan Thibodeaux <ryan.thibodeaux@...rlab.io>
> > > 
> > > The original timer slop value has not changed since the
> > > introduction
> > > of the Xen-aware Linux kernel code. This commit provides users an
> > > opportunity to tune timer performance given the refinements to
> > > hardware and the Xen event channel processing. It also mirrors
> > > a feature in the Xen hypervisor - the "timer_slop" Xen command
> > > line option.  
> > 
> > Is there any data that shows effects of using this new parameter?
> >   
> Yes, I've done some research and experiments on this. I did it
> together with a friend, which I'm Cc-ing, as I'm not sure we're
> ready/capable to share the results, yet (Luca?).

I think we can easily share the experimental data (cyclictest output
and plots).

Moreover, we can share the scripts and tools for running the
experiments (so, everyone can easily reproduce the numbers by simply
typing "make" and waiting for some time :)


I'll try to package the results and the scripts/tools this evening, and
I'll send them.



				Luca


> 
> What I think I can anticipate is that having such a high value for
> timer slop in the kernel, for the Xen clockevent device is (together
> with the also quite high default value of timer_slop in Xen itself)
> responsible for really high vcpu wakeup latencies.
> 
> Lowering those two values, reduces such latencies dramatically.
> 
> Regards,
> Dario