Re: [PATCH] printk: Print cpu number along with time

Jörn Engel
2014-04-24T21:24:09Z

On Thu, 24 April 2014 12:58:21 -0700, Greg Kroah-Hartman wrote:
> > @@ -868,11 +870,20 @@ static size_t print_time(u64 ts, char *buf)
> >
> > rem_nsec = do_div(ts, 1000000000);
> >
> > +#ifdef CONFIG_PRINTK_CPU
> > + if (!buf)
> > + return snprintf(NULL, 0, "[%5lu.000000,%02x] ",
>
> %02x for a cpu? What happens on machines with 8k cpus?

You should get something like:
Apr 23 10:34:03 [ 228.950926,1a] Call Trace:
Apr 23 10:34:03 [ 228.950926,201] Call Trace:
Apr 23 10:34:03 [ 228.950928,1a] [<ffffffff815e0f29>] schedule+0x29/0x70
...

That said, I don't have access to hardware with >256 cpus and haven't
actually tested this.

> And is this really an issue? Debugging by using printk is fun, but not
> really something that people need to add a cpu number to. Why not just
> use a tracepoint in your code to get the needed information instead?

Yes, this is an issue. There have been several instances in my life
when I would have had to guess which function belonged to which
backtrace without this patch. Good guesses take a long time, bad
guesses are nearly useless and giving up means you cannot debug the
issue.

I don't have a good example at hand, but here is something from a
quick grep for illustration.
Feb 22 19:04:46 [ 11.642470,2e] Call Trace:
Feb 22 19:04:46 [ 11.642472,16] Call Trace:
Feb 22 19:04:46 [ 11.642476,29] [<ffffffff81115bf8>] pcpu_alloc+0x988/0xa20
Feb 22 19:04:46 [ 11.642481,01] [<ffffffff810992ad>] ? find_symbol+0x3d/0xb0
Feb 22 19:04:46 [ 11.642489,05] [<ffffffff81115bf8>] pcpu_alloc+0x988/0xa20
Feb 22 19:04:46 [ 11.642493,12] [<ffffffff81115bf8>] pcpu_alloc+0x988/0xa20
Feb 22 19:04:46 [ 11.642498,08] [<ffffffff81115bf8>] pcpu_alloc+0x988/0xa20
Feb 22 19:04:46 [ 11.642504,21] [<ffffffff810992ad>] ? find_symbol+0x3d/0xb0

And are you seriously suggesting I use a tracepoint for kernel panics? ;)

Jörn

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