netdev - Re: [PATCH RFC] v1 expedited "big hammer" RCU grace periods

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Date:	Thu, 23 Apr 2009 09:54:36 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, davem@...emloft.net,
	dada1@...mosbay.com, zbr@...emap.net, jeff.chua.linux@...il.com,
	paulus@...ba.org, laijs@...fujitsu.com, jengelh@...ozas.de,
	r000n@...0n.net, benh@...nel.crashing.org,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [PATCH RFC] v1 expedited "big hammer" RCU grace periods


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> First cut of "big hammer" expedited RCU grace periods, but only 
> for rcu_bh.  This creates another softirq vector, so that entering 
> this softirq vector will have forced an rcu_bh quiescent state (as 
> noted by Dave Miller).  Use smp_call_function() to invoke 
> raise_softirq() on all CPUs in order to cause this to happen.  
> Track the CPUs that have passed through a quiescent state (or gone 
> offline) with a cpumask.
> 
> Does nothing to expedite callbacks already registered with 
> call_rcu_bh(), but there is no need to.
> 
> Shortcomings:
> 
> o	Untested, probably does not compile, not for inclusion.
> 
> o	Does not handle rcu, only rcu_bh.
> 
> Thoughts?

I'm wondering, why not just do a two-liner, along the lines of:

	for_each_online_cpu(cpu)
		smp_send_reschedule(cpu);

That should trigger a quiescent state on all online cpus. It wont 
perturb the scheduler state (which is reschedule-IPI invariant).
(And this is a big-hammer approach anyway so even if it did we 
wouldnt care.)

Am i missing something embarrasingly obvious perhaps?

	Ingo
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