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Date: Fri, 1 May 2015 18:34:32 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Rik van Riel <riel@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, williams@redhat.com,
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        Peter Zijlstra <peterz@infradead.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry
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* 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.

Thanks,

	Ingo