Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

[Rik van Riel]

On 05/01/2015 12:34 PM, Ingo Molnar wrote:
> 
> * Rik van Riel <riel@xxxxxxxxxx> 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.

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