Dan Magenheimer
2010-May-19 16:24 UTC
[Xen-devel] Concern about c/s 21404: Remove special-case paths for start-of-day SMP bringup
"Notably ganged TSC calibration is removed" Really? We have recently seen machines which generated the removed message "CPUX had N usecs TSC skew, fixed it up." I''m not sure I fully understand the scope or the purpose of all the patches, but since the TSC is the foundation for Xen system time which is the foundation for virtually all time handling in all guests (because even the non-TSC emulated timers are built on Xen system time), I think you are introducing a big risk here for an unknown number of existing systems. Some of the problems may be obvious because a guest often won''t boot when clocks are screwy, but other problems may not manifest until "later" when under a heavy load that causes more frequent vcpu->pcpu switching and may not be at all obviously related to clock problems. (We''ve recently seen that also with a Solaris HV guest running on a pre-4.0 Xen where rdtsc is not yet emulated.) Note that Linux is *adding* code to attempt to make TSC more reliable, while it appears this Xen patch is taking code away. If I understand, the purpose is to provide a better way to online CPUs so as to handle CPU hotplug better? While hotplug is a nice feature, am I correct that the feature is supported in a very small number of systems? And is actually used in an even smaller number? My two cents: Doesn''t seem like a good tradeoff to me... _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Keir Fraser
2010-May-19 19:15 UTC
[Xen-devel] Re: Concern about c/s 21404: Remove special-case paths for start-of-day SMP bringup
I''ll add an __initcall() to rendezvous the CPUs and whack their TSCs. K. On 19/05/2010 17:24, "Dan Magenheimer" <dan.magenheimer@oracle.com> wrote:> "Notably ganged TSC calibration is removed" > > Really? We have recently seen machines which generated > the removed message "CPUX had N usecs TSC skew, fixed it up." > > I''m not sure I fully understand the scope or the purpose > of all the patches, but since the TSC is the foundation for > Xen system time which is the foundation for virtually all time > handling in all guests (because even the non-TSC emulated > timers are built on Xen system time), I think you are introducing > a big risk here for an unknown number of existing systems. > Some of the problems may be obvious because a guest often > won''t boot when clocks are screwy, but other problems may > not manifest until "later" when under a heavy load that > causes more frequent vcpu->pcpu switching and may not > be at all obviously related to clock problems. (We''ve > recently seen that also with a Solaris HV guest running > on a pre-4.0 Xen where rdtsc is not yet emulated.) > > Note that Linux is *adding* code to attempt to make TSC more > reliable, while it appears this Xen patch is taking code away. > > If I understand, the purpose is to provide a better > way to online CPUs so as to handle CPU hotplug better? > While hotplug is a nice feature, am I correct that the > feature is supported in a very small number of > systems? And is actually used in an even smaller > number? > > My two cents: Doesn''t seem like a good tradeoff to me..._______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel