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

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Message-ID: <CAFm5wm0nf47bcTu7iiXA3geR_5dDTF1tnf81Lz7JzbWkKdhUtA@mail.gmail.com>
Date:	Fri, 3 Aug 2012 02:56:02 -0700
From:	"Pandita, Vikram" <vikram.pandita@...com>
To:	Kay Sievers <kay@...y.org>
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

Kay

On Fri, Aug 3, 2012 at 2:48 AM, Kay Sievers <kay@...y.org> wrote:
> 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.

When i was referring to 15 chars, its coming from here:
Its the space reserved in each line of output. Corresponding space for
cpuid is 4 chars: "[x] ":

static size_t print_time(u64 ts, char *buf)
{
        unsigned long rem_nsec;

        if (!printk_time)
                return 0;

        if (!buf)
>>>>                return 15;

>
> The CPU-ID would need at least two additional bytes (2^16 CPUS) in
> every record, unless it's a compile-time option.

are u proposing:
a) to make cpuid a u16?
b) to put cpuid in struct cont and struct log - under the #ifdef macro?

> I can't tell, if
> everybody wants to pay the storage space for the the CPU-ID feature.
>
> Kay
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