netdev - Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods

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Date:	Mon, 18 May 2009 17:42:41 +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] v5 expedited "big hammer" RCU grace periods


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

> > i might be missing something fundamental here, but why not just 
> > have per CPU helper threads, all on the same waitqueue, and wake 
> > them up via a single wake_up() call? That would remove the SMP 
> > cross call (wakeups do immediate cross-calls already).
> 
> My concern with this is that the cache misses accessing all the 
> processes on this single waitqueue would be serialized, slowing 
> things down. In contrast, the bitmask that smp_call_function() 
> traverses delivers on the order of a thousand CPUs' worth of bits 
> per cache miss.  I will give it a try, though.

At least if you go via the migration threads, you can queue up 
requests to them locally. But there's going to be cachemisses 
_anyway_, since you have to access them all from a single CPU, and 
then they have to fetch details about what to do, and then have to 
notify the originator about completion.

	Ingo
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