Messages in this thread | | | From | "Pandita, Vikram" <> | Date | Thu, 2 Aug 2012 16:50:08 -0700 | Subject | Re: [PATCH] printk: add option to print cpu id |
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Kay
On Thu, Aug 2, 2012 at 1:08 PM, Kay Sievers <kay@vrfy.org> wrote: > > On Thu, Aug 2, 2012 at 9:46 PM, Vikram Pandita <vikram.pandita@ti.com> > wrote: > > From: Vikram Pandita <vikram.pandita@ti.com> > > > > Introduce config option to enable CPU id reporting for printk() calls. > > > > 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. > > > > 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. > > > > Signed-off-by: Vikram Pandita <vikram.pandita@ti.com> > > Cc: Mike Turquette <mturquette@linaro.org> > > Cc: Vimarsh Zutshi <vimarsh.zutshi@gmail.com> > > --- > > kernel/printk.c | 20 ++++++++++++++++++++ > > lib/Kconfig.debug | 13 +++++++++++++ > > 2 files changed, 33 insertions(+) > > > > diff --git a/kernel/printk.c b/kernel/printk.c > > index 6a76ab9..50feb82 100644 > > --- a/kernel/printk.c > > +++ b/kernel/printk.c > > @@ -855,6 +855,25 @@ static size_t print_time(u64 ts, char *buf) > > (unsigned long)ts, rem_nsec / 1000); > > } > > > > +#if defined(CONFIG_PRINTK_CPUID) > > +static bool printk_cpuid = 1; > > +#else > > +static bool printk_cpuid; > > +#endif > > +module_param_named(cpuid, printk_cpuid, bool, S_IRUGO | S_IWUSR); > > + > > +static size_t print_cpuid(char *buf) > > +{ > > + > > + if (!printk_cpuid) > > + return 0; > > + > > + if (!buf) > > + return 4; > > + > > + return sprintf(buf, "[%1d] ", smp_processor_id()); > > +} > > + > > static size_t print_prefix(const struct log *msg, bool syslog, char > > *buf) > > { > > size_t len = 0; > > @@ -874,6 +893,7 @@ static size_t print_prefix(const struct log *msg, > > bool syslog, char *buf) > > } > > } > > > > + len += print_cpuid(buf ? buf + len : NULL); > > len += print_time(msg->ts_nsec, buf ? buf + len : NULL); > > return len; > > } > > How is that supposed to be useful? > > The prefix is added while exporting data from the kmsg buffer, which > is just the CPU that *reads* the data from the buffer, not the one > that has *written* the data it into it.
I don't think so. I can see the backtrace of the printk() call looks like follows:
print_cpuid print_prefix msg_print_text console_unlock vprint_emit printk
Now this is a synchronous path, where in the buffer is getting filled with cpuid and timer info from the printk() calling context. So you should get the right CPU id with the trace - with the exception that i pointed out earlier for preemption.
> > Do I miss anything here?
This is my understanding of the printk framework. At least the print_time and print_cpuid seems to be happening synchronously wrt printk
> > Thanks, > Kay
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