linux-kernel - Re: [PATCH 1/4] [x86] Add command line option to enable/disable hyper-threading.

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Date:	Thu, 30 Nov 2006 10:08:39 -0800
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Alan <alan@...rguk.ukuu.org.uk>
Cc:	Ben Collins <bcollins@...ntu.com>, linux-kernel@...r.kernel.org,
	torvalds@...l.org
Subject: Re: [PATCH 1/4] [x86] Add command line option to enable/disable hyper-threading.

On Thu, Nov 30, 2006 at 11:06:11AM +0000, Alan wrote:
> On Wed, 29 Nov 2006 23:26:05 -0500
> Ben Collins <bcollins@...ntu.com> wrote:
> 
> > This patch adds a config option to allow disabling hyper-threading by
> > default, and a kernel command line option to changes this default at
> > boot time.
> > 
> > Signed-off-by: Ben Collins <bcollins@...ntu.com>
> 
> The description is wrong - this does not disable hyperthreading it merely
> leaves one thread idle.

How does this patch achieve that? All this patch does is not detecting the
sibling topology. Kernel will still use all the threads and it just
forgoes the intelligence of which cpus are thread and core siblings and
thus disables the optimizations done by scheduler and doesn't export the
cpu topology to the user through sysfs and /proc.

Am I missing the point of this patch?

thanks,
suresh

> I don't believe Intel have ever published a
> procedure for truely disabling HT, but if you idle a thread you may want
> to adjust the cache settings on a PIV (10.5.6 in the intel docs) and set
> it to shared mode. Need to play more with what the bios does I guess.
> 
> So Ack but with the proviso it should say "Ignoring" or "Not using" not
> "Disabling", because it does not do the latter and there seem to be
> performance differences as a result
> 
> Acked-by: Alan Cox <alan@...hat.com>
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