From: luca abeni <luca.abeni@santannapisa.it>
To: Dario Faggioli <dfaggioli@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
thibodux@gmail.com, xen-devel@lists.xenproject.org,
linux-kernel@vger.kernel.org, oleksandr_andrushchenko@epam.com,
tglx@linutronix.de, jgross@suse.com, ryan.thibodeaux@starlab.io
Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option
Date: Tue, 26 Mar 2019 12:12:56 +0100 [thread overview]
Message-ID: <20190326121256.76ceed71@luca64> (raw)
In-Reply-To: <a32c8b1d7866ee3b074b1d674b40e03e51dbd54a.camel@suse.com>
Hi all,
On Tue, 26 Mar 2019 10:13:32 +0100
Dario Faggioli <dfaggioli@suse.com> wrote:
> On Mon, 2019-03-25 at 09:43 -0400, Boris Ostrovsky wrote:
> > On 3/25/19 8:05 AM, luca abeni wrote:
> > >
> > > 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?
> >
> So, AFAIUI, the thing is as follows. In clockevents_program_event(),
> we keep the delta between now and the next timer event within
> dev->max_delta_ns and dev->min_delta_ns:
>
> delta = min(delta, (int64_t) dev->max_delta_ns);
> delta = max(delta, (int64_t) dev->min_delta_ns);
>
> For Xen (well, for the Xen clock) we have:
>
> .max_delta_ns = 0xffffffff,
> .min_delta_ns = TIMER_SLOP,
>
> which means a guest can't ask for a timer to fire earlier than 100us
[...]
I know this is not fully related with the current discussion, but in
these days I had a look at the code again, and...
The comment for TIMER_SLOP in arch/x86/xen/time.c says:
/* Xen may fire a timer up to this many ns early */
Isn't the comment wrong? shouldn't it be "...many ns late" instead of
"early"?
Thanks,
Luca
next prev parent reply other threads:[~2019-03-26 11:13 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-22 18:29 [PATCH] x86/xen: Add "xen_timer_slop" command line option thibodux
2019-03-22 22:10 ` Boris Ostrovsky
2019-03-23 2:58 ` Dario Faggioli
2019-03-23 10:41 ` luca abeni
2019-03-25 12:05 ` luca abeni
2019-03-25 13:43 ` Boris Ostrovsky
2019-03-25 14:07 ` luca abeni
2019-03-25 14:11 ` Ryan Thibodeaux
2019-03-25 17:36 ` Ryan Thibodeaux
2019-03-25 18:31 ` Boris Ostrovsky
2019-03-26 9:13 ` Dario Faggioli
2019-03-26 11:12 ` luca abeni [this message]
2019-03-26 11:41 ` Ryan Thibodeaux
2019-03-26 23:21 ` Boris Ostrovsky
2019-03-27 10:00 ` Ryan Thibodeaux
2019-03-27 14:46 ` Boris Ostrovsky
2019-03-27 14:59 ` Ryan Thibodeaux
2019-03-27 15:19 ` Dario Faggioli
2019-03-23 12:00 ` Ryan Thibodeaux
2019-03-24 18:07 ` Boris Ostrovsky
2019-03-25 10:36 ` Dario Faggioli
2019-04-24 18:47 ` Boris Ostrovsky
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