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

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Message-ID: <20090518160254.GD6768@linux.vnet.ibm.com>
Date:	Mon, 18 May 2009 09:02:54 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
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

On Mon, May 18, 2009 at 05:42:41PM +0200, Ingo Molnar wrote:
> 
> * 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.

Ah, so you are suggesting that I use smp_call_function() to run code on
each CPU that wakes up that CPU's migration thread?  I will take a look
at this.

							Thanx, Paul
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