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Date: Fri, 23 Aug 2002 09:10:33 +0100 (BST)
From: Hugh Dickins <hugh@veritas.com>
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To: Gilad Ben-Yossef <gilad@benyossef.com>
cc: James Bourne <jbourne@mtroyal.ab.ca>,
       "Reed, Timothy A" <timothy.a.reed@lmco.com>,
       <linux-kernel@vger.kernel.org>
Subject: Re: Hyperthreading
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On 23 Aug 2002, Gilad Ben-Yossef wrote:
> 
> hmm... isn't there an option to tell the kernel you are using a
> HyperThreaded system, or is it detected on runtime?  I mean, think about
> a P4 Xeon 2 way SMP - unless told otherwise the kernel will 'see' it as
> a 4 way SMP box *but* the proccessors are not equel!

There is the "noht" boot arg to tell the kernel not to use HT,
and there is the X86_FEATURE_HT cpu capability flag, but I think
the runtime indication you're looking for is smp_num_siblings:
1 without HT, 2 (ever more?) with HT.

> If for example, you have a task running and another task just woke up
> and the scheduler needs to assign a CPU for it, choosing the other
> 'instance' of the same CPU as the already running task to run it on as
> opposed to choosing one of the 'instanaces' of the other seperate CPU
> seems a mistake IMHO, but the scheduler won't be able to make the
> judgment because it doesn't know it is running on a SMT box at all.

SMT I don't know.  Your point is valid, and you'll find the 2.4 -aa
and -ac trees (both using newer O1 scheduler) each have code in (or
called out from) kernel/sched.c to deal with that.

The mainline 2.4 does not take that into consideration, and so far
as I can see (please correct me), nor does 2.5 as yet - will probably
get added from 2.4 -aa or -ac in due course.  It's not an issue of
correctness, just optimality.

Hugh

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