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

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Message-ID: <20150501163431.GB1327@gmail.com>
Date:	Fri, 1 May 2015 18:34:32 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry


* Rik van Riel <riel@...hat.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
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