linux-kernel - Re: [RFC 4/4]x86: avoid tlbstate lock if no enough cpus

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Message-ID: <1288769123.2467.681.camel@edumazet-laptop>
Date:	Wed, 03 Nov 2010 08:25:23 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [RFC 4/4]x86: avoid tlbstate lock if no enough cpus

Le mercredi 03 novembre 2010 à 15:19 +0800, Shaohua Li a écrit :
> On Wed, 2010-11-03 at 15:12 +0800, Eric Dumazet wrote:
> > Le mercredi 03 novembre 2010 à 15:06 +0800, Shaohua Li a écrit :
> > > just don't want to include the non-present cpus here. I wonder why we
> > > haven't a variable to record online cpu number.
> > 
> > What prevents a 256 cpus machine, to have 8 online cpus that all use the
> > same TLB vector ?
> > 
> > (Max 32 vectors, so 8 cpus share each vector, settled at boot time)
> > 
> > Forget about 'online', and think 'possible' ;)
> Hmm, the spread vectors to node already merged, how could the 8 cpus
> share a vector?
> 

You boot a machine with 256 cpu.

They are online and very well.

Each vector is shared by at least 8 cpus, because 256/32 = 8. OK ?

Now you off-line 256-8 cpus, because you have HOTPLUG capability in your
kernel and you have some policy to bring them up later if needed.

What happens ? Do you rebalance TLB vectors to make sure each cpu has
its own vector ?



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