[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [BUG 1282] time jump on live migrate root cause & proposed fixes
Hi, I have done some debugging to find out the root cause of bug 1282, which has the following symptoms with paravirtualized guests: - after a live migrate, the time on the guest can jump - after a live migrate, the guest "forgets" to wake up processes - after a domU save, dom0 reboot and domU restore, the time is correct but processes are not woken up from sys_nanosleep The problem seems to stem from the fact that domU uses the hypervisor's system_time, which is the time since hypervisor system bootup in nanoseconds, as its base for timekeeping. This works fine as long as the guest stays on the same hypervisor, but if the guest is migrated to a hypervisor with a different uptime, problems ensue. Specifically, if the guest is migrated to a host with a lower uptime, processes that call sys_nanosleep() will not be woken up until the new host's uptime catches up with the uptime of the old host! While waiting for the uptime to catch up, gettimeofday always returns the same value. Conversely, if a guest migrates from a host with a lower uptime to a host with a higher uptime, the system time in the guest advances by the difference between the two uptimes. I can think of a few possible fixes for this issue: 1) have system_time in the hypervisor start at unix epoch 0 (january 1st 1970) instead of at boot time - this may require some magic to sync_cmos_clock(), sync_xen_wallclock() and/or other functions so dom0 does not get too confused while changing the time during bootup 2) have time_init() and time_resume() calculate the hypervisor boot time from the shared_info ->wc_sec ->wc_nsec and the shared_info->per cpu vcpu_info->system_time -- if the host boot time changes (by more than a second?) adjust some local offset that we add into get_nsec_offset() and get_usec_offset() to always adjust the time right 3) get_time_values_from_xen() and __update_wallclock() can keep track of such an offset by themselves Does anybody have comments on the ideas above, or maybe even better ideas on how to fix the problem? :) -- All Rights Reversed _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |