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[Xen-devel] Re: xen_sched_clock problem in RHEL6b2, Was: [PATCH] x86: unconditionally mark TSC unstable under Xen
Hi Dan,
since it looks like this is the kernel/hypervisor bug I'm not really the
right one to ask. You should better as Andrew Jones - CCed now (his
e-mail is drjones@xxxxxxxxxx just for sure) - since he's working in the
kernel-xen area and therefore even the RHEL-6 kernel so I guess he's a
good guy to ask about this one. Also, according to the links it seems
you're running RHEL-6 beta 2 as PV guest, right ? Are you running this
on Xen-4.1-unstable or what version?
Andrew, could you please have a look this one? I don't know whether it's
RHEL-6 kernel bug itself or whether there's some bug in the hypervisor,
nevetheless both parts belongs to the same component - kernel-xen -
you're working on so I guess you're a good guy to ask. Please, leave me
in the CC list as I'm interested as well.
Thanks,
Michal
On 07/17/2010 12:32 AM, Dan Magenheimer wrote:
Hi Michael --
Not sure if you are the right person for this, but FYI
it appears that this bug is also in RHEL6 beta 2 and
might account for some weird issues I've seen running
RHEL6 betas as Xen guests.
Problem description:
http://lists.xensource.com/archives/html/xen-devel/2010-07/msg00241.html
Jeremy's solution (forwarded below):
http://lists.xensource.com/archives/html/xen-devel/2010-07/msg00749.html
Dan
-----Original Message-----
From: Jeremy Fitzhardinge [mailto:jeremy@xxxxxxxx]
Sent: Wednesday, July 14, 2010 3:36 PM
To: Jed Smith
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Jan Beulich
Subject: [Xen-devel] Re: [PATCH] x86: unconditionally mark TSC unstable
under Xen
On 07/14/2010 12:24 PM, Jed Smith wrote:
Jeremy, Jan - what do you think? Is this a bad move? I feel like
there
is a consequence to this that I am unaware of, but it fixes my issue.
Ah, well that's interesting.
There's a couple of comments:
1. you can't do this with just a compile-time test, since the same
kernel can also boot native
2. nothing in a Xen PV domU environment should be using the tsc
directly, so this shouldn't have an effect. If something is
using
the tsc we should track it down.
I wonder, however, if you're getting the same result as Jan's
suggestion
of making sched_clock unstable by making the tsc unstable.
In that case, this patch may help:
Subject: [PATCH] xen: disable xen_sched_clock by default
xen_sched_clock only counts unstolen time. In principle this should
be useful to the Linux scheduler so that it knows how much time a
process
actually consumed. But in practice this doesn't work very well as the
scheduler expects the sched_clock time to be synchronized between
cpus. It also uses sched_clock to measure the time a task spends
sleeping, in which case "unstolen time" isn't meaningful.
So just use plain xen_clocksource_read to return wallclock nanoseconds
for sched_clock.
Signed-off-by: Jeremy Fitzhardinge<jeremy.fitzhardinge@xxxxxxxxxx>
diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
index b83e119..6a09f01 100644
--- a/arch/x86/xen/Kconfig
+++ b/arch/x86/xen/Kconfig
@@ -29,6 +29,10 @@ config XEN_SAVE_RESTORE
depends on XEN&& PM
default y
+config XEN_SCHED_CLOCK
+ bool
+ default n
+
config XEN_DEBUG_FS
bool "Enable Xen debug and tuning parameters in debugfs"
depends on XEN&& DEBUG_FS
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 4b57c0b..242a230 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -939,7 +939,11 @@ static const struct pv_time_ops xen_time_ops
__initdata = {
.set_wallclock = xen_set_wallclock,
.get_wallclock = xen_get_wallclock,
.get_tsc_khz = xen_tsc_khz,
+#ifdef CONFIG_XEN_SCHED_CLOCK
.sched_clock = xen_sched_clock,
+#else
+ .sched_clock = xen_clocksource_read,
+#endif
};
static const struct pv_cpu_ops xen_cpu_ops __initdata = {
diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 9d1f853..32dc3dc 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -154,6 +154,7 @@ static void do_stolen_accounting(void)
account_idle_ticks(ticks);
}
+#ifdef CONFIG_XEN_SCHED_CLOCK
/*
* Xen sched_clock implementation. Returns the number of unstolen
* nanoseconds, which is nanoseconds the VCPU spent in RUNNING+BLOCKED
@@ -191,7 +192,7 @@ unsigned long long xen_sched_clock(void)
return ret;
}
-
+#endif
/* Get the TSC speed from Xen */
unsigned long xen_tsc_khz(void)
Thanks,
J
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--
Michal Novotny<minovotn@xxxxxxxxxx>, RHCE
Virtualization Team (xen userspace), Red Hat
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