From: Atsushi SAKAI <sakaia@jp.fujitsu.com>
Subject: [PATCH][RFC] remove PV/x86 warnings (time went
	backwards)
Date: Thu, 11 Dec 2008 17:02:46 +0900
Message-ID: <20081211080251.02F901808C@m021.s.css.fujitsu.com>
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Hi,

This patch intends to remove warnings for Time went backwards
(negative delta) like follows.
========from Xen BZ 195
Timer ISR/0: Time went backwards: delta=-10048465 cpu_delta=9951535
shadow=706605145 0: 71640046095
========

Currently (static u64) processed_system_time@time-xen.c
 uses for calculating jiffies and wall_clock.
and the processed_system_time saves maximum vcpu time value of per_cpu(processed_system_time).

At timer_interrupt@time-xen.c,
they calculate "delta" compared to vcpu time(shadow).
sometimes the value goes to negative.
This is because vcpu time is varies.(scheduler etc.)

This patch just removes the warnings of PV/x86,
(Other way is loosen "delta" checking.(current checking is < -10msec)
 Or vcpu synchronization is needed.)

This problem occurs on current linux-2.6.18-xen tree only.
Not occured on pv_ops kernel.

Of course, delta_cpu comparison keeps 
since it is just compares per_cpu variable only.

==for paravirt_ops ==
For paravirt_ops, time.c is completely changed.
http://git.kernel.org/?p=linux/kernel/git/jeremy/xen.git;a=blob;f=arch/x86/xen/time.c;h=c9f7cda48ed78ecbe1b421c540bbe2b777681400;hb=HEAD

See also
2006/07(which uses delta)
http://lists.xensource.com/archives/html/xen-devel/2006-07/msg00567.html
2007/03(which does not uses delta)
http://lists.xensource.com/archives/html/xen-devel/2007-03/msg00554.html


== Related Bugzilla ==
This patch related to following bugzilla.

XenBugzilla:
Many Timer ISR: Time went backwards logs
http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=195

RedHat Bugzilla:
Many "Timer ISR: Time went backwards" after booting xen kernel
https://bugzilla.redhat.com/show_bug.cgi?id=222973


Signed-off-by: Atsushi SAKAI <sakaia@jp.fujitsu.com>

Thanks
Atsushi SAKAI


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