Re: [v7 0/8] Reduce cross CPU IPI interference

From: Gilad Ben-Yossef
Date: Sun Feb 05 2012 - 07:06:38 EST

On Thu, Feb 2, 2012 at 7:25 PM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> On Thu, 2 Feb 2012, Paul E. McKenney wrote:
>> Frederic's work checks to see if there is only one runnable user task
>> on a given CPU.  If there is only one, then the scheduling-clock interrupt
>> is turned off for that CPU, and RCU is told to ignore it while it is
>> executing in user space.  Not sure whether this covers KVM guests.
>> In any case, this is not yet in mainline.
> Sounds great. Is there any plan on when to merge it? Where are the most up
> to date patches vs mainstream?

Frederic has the latest version in a git tree here:


It's on top latest rcu/core.

I've been playing with it for some time now. It works very well, considering the
early state - there are a couple of TODO items listed here: and I've seen an assert from
the RCU code once.

Also, there is some system stuff "in the way" so to speak, of getting the full

I had to disable the clock source watchdog (I'm testing in a KVM VM, so I guess
the TSC is not stable), the vmstat_stats work on that CPU and to (try
to) fix what
looks like a bug in the NOHZ timer code.

But the good news is that with these hacks applied I managed to run a 100%
CPU task with zero interrupts (ticks or otherwise) on an isolated cpu.

Disregarding TLB overhead, you get bare metal performance with Linux user
space manageability and debug capabilities. Pretty magical really: It's like
eating your cake and having it too :-)


Gilad Ben-Yossef
Chief Coffee Drinker

"If you take a class in large-scale robotics, can you end up in a
situation where the homework eats your dog?"
 -- Jean-Baptiste Queru
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