linux-kernel - Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi

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Date:	Wed, 06 Jun 2012 10:42:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Luck, Tony" <tony.luck@...el.com>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>, H Peter Anvin <hpa@...or.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	Arjan Dan De Ven <arjan@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	x86 <x86@...nel.org>, linux-pm <linux-pm@...r.kernel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or
 nmi

On Tue, 2012-06-05 at 15:12 -0700, Paul E. McKenney wrote:
> > RCU has similar nasties.
> 
> I am working to rid RCU of this sort of thing.  I have rcu_barrier() so
> that it avoids messing with CPUs that don't have callbacks, which will
> be almost all of the idle CPUs, especially for CONFIG_RCU_FAST_NO_HZ=y.
> I believe that I have also removed all of RCU's dependencies on CPU
> hotplug's using kstopmachine, though Murphy would say otherwise.
> 
> I still need to fix up synchronize_sched_expedited(), but that is on
> the list.  I considered getting rid of this one, but I am probably going
> to have to make synchronize_sched() map to it during boot time to keep
> the boot-speed demons satisfied. 

Not the point really. Its perfectly fine for applications in an
'isolated' set to use system calls, hence they get to participate in RCU
state.

I don't think the isolation means userspace while(1) applications is
interesting. Sure, some people do this, and we should dtrt for them, but
the far more interesting case is 'regular' applications that do use
system calls.
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