linux-kernel - Re: [PATCH] perf_events: fix bug in AMD per-cpu initialization

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Date:	Thu, 18 Mar 2010 01:33:15 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com, robert.richter@....com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com
Subject: Re: [PATCH] perf_events: fix bug in AMD per-cpu initialization

On Thu, Mar 18, 2010 at 12:47 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2010-03-17 at 10:40 +0200, Stephane Eranian wrote:
>>       On AMD processors, we need to allocate a data structure per Northbridge
>>       to handle certain events.
>>
>>       On CPU initialization, we need to query the Northbridge id and check
>>       whether the structure is already allocated or not. We use the
>>       amd_get_nb_id() function to request the Northbridge identification.
>>
>>       The recent cleanup of the CPU online/offline initialization introduced
>>       a bug. AMD cpu initialization is invoked on CPU_UP_PREPARE callback.
>>       This is before the CPU Northbridge id is calculated. Therefore no
>>       processor had a Northbridge structure allocated except the boot
>>       processor. That was causing bogus NB event scheduling.
>>
>>       This patch uses the CPU_ONLINE callback to initialize the AMD
>>       Northbridge structure. This way amd_get_nb_id() returns valid
>>       information.
>>
>>       The x86_cpu_up() callback was added. Could not call it cpu_online
>>       because of existing macro.
>>
>>       Signed-off-by: Stephane Eranian <eranian@...gle.com>
>
>
> No, ONLINE is not exposed for a good reason, its always wrong.
>
> Use prepare to allocate data, and starting to initialize stuff on the
> cpu proper once its up. Online is after the cpu is up and running and we
> are already scheduling stuff on it so its too late to initialize things.
>
>
I can't because amd_get_nb_id() returns garbage for the CPU
when you call from CPU_STARTING. So looks like initialization
of cpu_llc_id needs to be moved way earlier. I don't want to have
to duplicate that code.
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