linux-kernel - Re: [patch] genapic: optimize & fix APIC mode setup

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Message-Id: <200611111620.24551.ak@suse.de>
Date:	Sat, 11 Nov 2006 16:20:24 +0100
From:	Andi Kleen <ak@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	ashok.raj@...el.com
Subject: Re: [patch] genapic: optimize & fix APIC mode setup

On Saturday 11 November 2006 16:14, Ingo Molnar wrote:
> From: Ingo Molnar <mingo@...e.hu>
> 
> this patch fixes a couple of inconsistencies/problems i found while 
> reviewing the x86_64 genapic code (when i was chasing mysterious eth0 
> timeouts that would only trigger if CPU_HOTPLUG is enabled):
> 
>  - AMD systems defaulted to the slower flat-physical mode instead
>    of the flat-logical mode. The only restriction on AMD systems
>    is that they should not use clustered APIC mode.

This will open a race on CPU hotunplug unfortunately
(common for multi core suspend) 

> 
>  - removed the CPU hotplug hacks, switching the default for small
>    systems back from phys-flat to logical-flat. The switching to logical 
>    flat mode on small systems fixed sporadic ethernet driver timeouts i 
>    was getting on a dual-core Athlon64 system:

That will break CPU hotplug on some Intel systems (Ashok can give details) 

That is caused ethernet timeouts is weird, probably needs to be root caused.

>     NETDEV WATCHDOG: eth0: transmit timed out
>     eth0: Transmit timeout, status 0c 0005 c07f media 80.
>     eth0: Tx queue start entry 32  dirty entry 28.
>     eth0:  Tx descriptor 0 is 0008a04a. (queue head)
>     eth0:  Tx descriptor 1 is 0008a04a.
>     eth0:  Tx descriptor 2 is 0008a04a.
>     eth0:  Tx descriptor 3 is 0008a04a.
>     eth0: link up, 100Mbps, full-duplex, lpa 0xC5E1
> 
>  - The use of '<= 8' was a bug by itself (the valid APIC ids
>    for logical flat mode go from 0 to 7, not 0 to 8). The new logic
>    is to use logical flat mode on both AMD and Intel systems, and
>    to only switch to physical mode when logical mode cannot be used.
>    If CPU hotplug is racy wrt. APIC shutdown then CPU hotplug needs
>    fixing, not the whole IRQ system be made inconsistent and slowed
>    down.

Yes that needs to be fixed.

-Andi
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