Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry | From | Mike Galbraith <> | Date | Sat, 02 May 2015 06:06:21 +0200 |
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On Fri, 2015-05-01 at 14:05 -0400, Rik van Riel wrote: > On 05/01/2015 12:34 PM, Ingo Molnar wrote: > > > > * Rik van Riel <riel@redhat.com> 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.
I don't get it. How the heck do people manage to talk about realtime in virtual boxen, and not at least crack a smile. Real, virtual, real, virtual... what's wrong with this picture?
Why is virtual realtime not an oxymoron?
I did that for grins once, and it was either really funny, or really sad, not sure which... but it did not look really really useful.
-Mike
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