On Thursday 11 March 2010 15:50:54 Avi Kivity wrote: > On 03/11/2010 09:46 AM, Sheng Yang wrote: > > On Thursday 11 March 2010 15:36:01 Avi Kivity wrote: > >> On 03/11/2010 09:20 AM, Sheng Yang wrote: > >>> Currently we can only get the cpu_stat of whole guest as one. This > >>> patch enhanced cpu_stat with more detail, has guest_system and > >>> guest_user cpu time statistics with a little overhead. > >>> > >>> Signed-off-by: Sheng Yang<sheng@xxxxxxxxxxxxxxx> > >>> --- > >>> > >>> This draft patch based on KVM upstream to show the idea. I would split > >>> it into more kernel friendly version later. > >>> > >>> The overhead is, the cost of get_cpl() after each exit from guest. > >> > >> This can be very expensive in the nested virtualization case, so I > >> wouldn't like this to be in normal paths. I think detailed profiling > >> like that can be left to 'perf kvm', which only has overhead if enabled > >> at runtime. > > > > Yes, that's my concern too(though nested vmcs/vmcb read already too > > expensive, they should be optimized...). > > Any ideas on how to do that? Perhaps use paravirt_ops to covert the > vmread into a memory read? We store the vmwrites in the vmcs anyway. When Qing(CCed) was working on nested VMX in the past, he found PV vmread/vmwrite indeed works well(it would write to the virtual vmcs so vmwrite can also benefit). Though compared to old machine(one our internal patch shows improve more than 5%), NHM get less benefit due to the reduced vmexit cost. -- regards Yang, Sheng > > > The other concern is, perf alike mechanism would > > bring a lot more overhead compared to this. > > Ordinarily users won't care if time is spent in guest kernel mode or > guest user mode. They want to see which guest is imposing a load on a > system. I consider a user profiling a guest from the host an advanced > and rarer use case, so it's okay to require tools and additional > overhead for this. > > >> For example you can put the code to note the cpl in a tracepoint which > >> is enabled dynamically. > > > > Yanmin have already implement "perf kvm" to support this. We are just > > arguing if a normal top-alike mechanism is necessary. > > > > I am also considering to make it a feature that can be disabled. But > > seems it make things complicate and result in uncertain cpustat output. > > I'm not even sure that guest time was a good idea. > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
