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

[Pandita, Vikram]

Venu

On Fri, Aug 3, 2012 at 2:32 AM, Venu Byravarasu <vbyravarasu@xxxxxxxxxx> wrote:
>
>> -----Original Message-----
>> From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-
>> owner@xxxxxxxxxxxxxxx] On Behalf Of Venu Byravarasu
>> Sent: Friday, August 03, 2012 2:58 PM
>> To: Vikram Pandita; gregkh@xxxxxxxxxxxxxxxxxxx; kay@xxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx; Mike Turquette; Vimarsh Zutshi
>> Subject: RE: [PATCH v2] printk: add option to print cpu id
>>
>> > -----Original Message-----
>> > From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-
>> > owner@xxxxxxxxxxxxxxx] On Behalf Of Vikram Pandita
>> > Sent: Friday, August 03, 2012 2:46 PM
>> > To: gregkh@xxxxxxxxxxxxxxxxxxx; kay@xxxxxxxx
>> > Cc: linux-kernel@xxxxxxxxxxxxxxx; Vikram Pandita; Mike Turquette; Vimarsh
>> > Zutshi
>> > Subject: [PATCH v2] printk: add option to print cpu id
>> >
>> > From: Vikram Pandita <vikram.pandita@xxxxxx>
>> >
>> > Introduce config option to enable CPU id reporting for printk() calls.
>> >
>> > Example logs with this option enabled look like:
>> >  [1] [    2.328613] usbcore: registered new interface driver libusual
>> >  [1] [    2.335418] usbcore: registered new interface driver usbtest
>> >  [1] [    2.342803] mousedev: PS/2 mouse device common for all mice
>> >  [0] [    2.352600] twl_rtc twl_rtc: Power up reset detected.
>> >  [0] [    2.359191] twl_rtc twl_rtc: Enabling TWL-RTC
>> >  [1] [    2.367797] twl_rtc twl_rtc: rtc core: registered twl_rtc as rtc0
>> >  [1] [    2.375274] i2c /dev entries driver
>> >  [1] [    2.382324] Driver for 1-wire Dallas network protocol.
>> >
>> > Its sometimes very useful to have printk also print the CPU Identifier
>> > that executed the call. This has helped to debug various SMP issues on
>> > shipping
>> > products.
>>
>
> 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?


>
>> Is it not better to have macros which will have wrapper to printk with
>> required
>> debug info added? E.g. cpu# in your case.
>> If by default we add cupid, is it not over head in each message getting
>> printed with printk?
>>
>> >
>> > Known limitation is if the system gets preempted between function call and
>> > actual printk, the reported cpu-id might not be accurate. But most of the
>> > times its seen to give a good feel of how the N cpu's in the system are
>> > getting loaded.
>> >
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