On Tue, Dec 17, 2013 at 8:43 PM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote: > On 2013-12-17 12:31, Arthur Chunqi Li wrote: >> Hi Jan, >> >> On Tue, Dec 17, 2013 at 7:21 PM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote: >>> On 2013-12-17 10:32, Arthur Chunqi Li wrote: >>>> Hi Jan and Paolo, >>>> >>>> I've tried to use preemption timer in KVM to trap vcpu regularly, but >>>> there's something unexpected. I run a VM with 4 vcpus and give them >>>> the same preemption timer value (e.g. 1000000) with all bits set >>>> (activate/save bits), then reset the value in preemption time-out >>>> handler. >>>> >>>> Thus I expected these vcpus trap regularly in some special turns. But >>>> I found that when the VM is not busy, some vcpus are trapped much less >>>> frequently than others. In Intel SDM, I noticed that preemption timer >>>> is only related to TSC, and I think all the vcpus should trap in a >>>> similar frequency. >>>> >>>> Could u help me explain this phenomenon? >>> >>> Are you on a CPU that has non-broken preemption timer support? Anything >>> prior Haswell is known to tick with arbitrary frequencies. >> >> My CPU is Intel(R) Xeon(R) CPU E5620 @ 2.40GHz. > > Hmm, this one seems unaffected. Didn't find a "specification update". > Just like Paolo asked: Your original test case passes? > >> >> Besides, what do you mean by arbitrary frequencies? > > On older CPUs, the tick rate of the preemption timer does not correlate > with the TSC, definitely not in the way the spec defined. > > Back to your original question: Are we talking about native use of the > preemption timer via a patched KVM or nested use inside a KVM virtual > machine? It is about the native use. I think it may due to the scheduling. When vcpu is scheduled out of pcpu, will the preemption timer work still? Oh, another problem, I use the released kernel 3.11, not the latest one. Does this matter? Arthur > > Jan > > -- > Siemens AG, Corporate Technology, CT RTC ITP SES-DE > Corporate Competence Center Embedded Linux -- Arthur Chunqi Li Department of Computer Science School of EECS Peking University Beijing, China -- 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
