On 05/01/2015 12:34 PM, Ingo Molnar wrote: > > * Rik van Riel <[email protected]> wrote: > >>> I can understand people running hard-RT workloads not wanting to >>> see the overhead of a timer tick or a scheduler tick with variable >>> (and occasionally heavy) work done in IRQ context, but the jitter >>> caused by a single trivial IPI with constant work should be very, >>> very low and constant. >> >> Not if the realtime workload is running inside a KVM guest. > > I don't buy this: > >> At that point an IPI, either on the host or in the guest, involves a >> full VMEXIT & VMENTER cycle. > > So a full VMEXIT/VMENTER costs how much, 2000 cycles? That's around 1 > usec on recent hardware, and I bet it will get better with time. > > I'm not aware of any hard-RT workload that cannot take 1 usec > latencies.
Now think about doing this kind of IPI from inside a guest, to another VCPU on the same guest. Now you are looking at VMEXIT/VMENTER on the first VCPU, plus the cost of the IPI on the host, plus the cost of the emulation layer, plus VMEXIT/VMENTER on the second VCPU to trigger the IPI work, and possibly a second VMEXIT/VMENTER for IPI completion. I suspect it would be better to do RCU callback offload in some other way. -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
