linux-kernel - Re: [PATCH v2] printk: add option to print cpu id

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Date:	Fri, 3 Aug 2012 11:48:26 +0200
From:	Kay Sievers <kay@...y.org>
To:	"Pandita, Vikram" <vikram.pandita@...com>
Cc:	Venu Byravarasu <vbyravarasu@...dia.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>,
	Vimarsh Zutshi <vimarsh.zutshi@...il.com>
Subject: Re: [PATCH v2] printk: add option to print cpu id

On Fri, Aug 3, 2012 at 11:36 AM, Pandita, Vikram <vikram.pandita@...com> wrote:
> On Fri, Aug 3, 2012 at 2:32 AM, Venu Byravarasu <vbyravarasu@...dia.com> wrote:

>> As having Macro locally in the file of interest would serve the purpose,
>> Why to change the printk code?
>
> As stated, the logic followed is exactly similar to well proven and
> approved way to handle printk time stamp: CONFIG_PRINTK_TIME
> Its an overhead on the system that enables the config option:
> CONFIG_PRINTK_CPUID
> Otherwise the system remains as is.
>
> To gain insight on SMP system logging behavior, the price to pay is
> the extra 4 chars per printk line,
> just like printk-time adds 15 chars to each line. Both options can be
> disabled as desired.
>
> So i am not sure what kind of option you are proposing?
> Do u imply PRINTK_TIME is not right then?

It's 8 bytes per message for storing the timestamp in the records.
There is never 15 bytes storage space needed, the prefix is
constructed on-the-fly only while exporting the data.

The CPU-ID would need at least two additional bytes (2^16 CPUS) in
every record, unless it's a compile-time option. I can't tell, if
everybody wants to pay the storage space for the the CPU-ID feature.

Kay
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