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

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Message-ID: <20190325150703.3774b92d@luca64>
Date:   Mon, 25 Mar 2019 15:07:03 +0100
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Dario Faggioli <dfaggioli@...e.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,

On Mon, 25 Mar 2019 09:43:20 -0400
Boris Ostrovsky <boris.ostrovsky@...cle.com> wrote:
[...]
> > http://retis.santannapisa.it/luca/XenTimers/
> > (there also is a link to the scripts to be used for reproducing the
> > results). The latencies have been measured by running cyclictest in
> > the guest (see the scripts for details).
> >
> > The picture shows the latencies measured with an unpatched guest
> > kernel and with a guest kernel having TIMER_SLOP set to 1000
> > (arbitrary small value :).
> > All the experiments have been performed booting the hypervisor with
> > a small timer_slop (the hypervisor's one) value. So, they show that
> > decreasing the hypervisor's timer_slop is not enough to measure low
> > latencies with cyclictest.  
> 
> 
> 
> I have a couple of questions:
> * Does it make sense to make this a tunable for other clockevent
> devices as well?
> * This patch adjusts min value. Could max value (ever) need a similar
> adjustment?

Sorry, I do not know much about clockevent devices, so I have no answers
to these questions...

What I can say is that when I repeated the cyclictest experiments on
VMs using a different clockevent device (lapic) I did not measure large
latencies.
So, I guess the "lapic" clockevent device already defaults to a smaller
min value (not sure about other clockevent devices, I do not know how
to test them).



				Luca