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SubjectRe: [PATCH] printk: add printk_delay to make messages readable for some scenarios
On Tue, 16 Jun 2009 16:54:12 +0800 Dave Young <hidave.darkstar@gmail.com> wrote:

> When syslog is not possible, at the same time there's no serial/net console
> available, it will be hard to read the printk messages.
> For example oops/panic/warning messages in shutdown phase.
>
> Here add a printk delay feature, we can make each printk message
> delay some milliseconds.
>
> Setting the delay by proc/sysctl interface: /proc/sys/kernel/printk_delay
>
> The value range from 0 - 10000, default value is 0
>

Seems OK(ish).

> ---
> Documentation/sysctl/kernel.txt | 8 ++++++++
> include/linux/kernel.h | 2 ++
> include/linux/sysctl.h | 1 +
> kernel/printk.c | 14 ++++++++++++++
> kernel/sysctl.c | 12 ++++++++++++
> 5 files changed, 37 insertions(+)
>
> --- linux-2.6.orig/kernel/printk.c 2009-06-16 11:07:15.000000000 +0800
> +++ linux-2.6/kernel/printk.c 2009-06-16 11:31:01.000000000 +0800
> @@ -638,6 +638,19 @@ static int recursion_bug;
> static int new_text_line = 1;
> static char printk_buf[1024];
>
> +int printk_delay_msec;
> +
> +static inline void printk_delay(void)
> +{
> + if (unlikely(printk_delay_msec)) {
> + int m = printk_delay_msec;
> + while (m--) {
> + mdelay(1);
> + touch_nmi_watchdog();
> + }
> + }
> +}
> +
> asmlinkage int vprintk(const char *fmt, va_list args)
> {
> int printed_len = 0;
> @@ -647,6 +660,7 @@ asmlinkage int vprintk(const char *fmt,
> char *p;
>
> boot_delay_msec();
> + printk_delay();
>
> preempt_disable();
> /* This stops the holder of console_sem just where we want him */
> --- linux-2.6.orig/include/linux/kernel.h 2009-04-18 19:48:06.000000000 +0800
> +++ linux-2.6/include/linux/kernel.h 2009-06-16 11:24:43.000000000 +0800
> @@ -243,6 +243,8 @@ extern int printk_ratelimit(void);
> extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
> unsigned int interval_msec);
>
> +extern int printk_delay_msec;
> +

Sigh.

It's pretty dumb to clutter up kernel.h just because we want to share a
declaration with sysctl.c. And it was pretty dumb to fill sysctl.c with
extern declarations.

What we should do is to create a new header file which contains all the
declarations which sysctl.c needs. Include that from within sysctl.c
and all .c files which which to share declarations with sysctl.c.

But that's all a matter for another patchset.

> /*
> * Print a one-time message (analogous to WARN_ONCE() et al):
> */
> --- linux-2.6.orig/kernel/sysctl.c 2009-06-15 10:33:11.000000000 +0800
> +++ linux-2.6/kernel/sysctl.c 2009-06-16 11:29:52.000000000 +0800
> @@ -102,6 +102,7 @@ static int __maybe_unused one = 1;
> static int __maybe_unused two = 2;
> static unsigned long one_ul = 1;
> static int one_hundred = 100;
> +static int ten_thousand = 10000;
>
> /* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
> static unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
> @@ -699,6 +700,17 @@ static struct ctl_table kern_table[] = {
> .mode = 0644,
> .proc_handler = &proc_dointvec,
> },
> + {
> + .ctl_name = KERN_PRINTK_DELAY,
> + .procname = "printk_delay",
> + .data = &printk_delay_msec,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = &proc_dointvec_minmax,
> + .strategy = &sysctl_intvec,
> + .extra1 = &zero,
> + .extra2 = &ten_thousand,
> + },
> #endif

See that endif there? If CONFIG_PRINTK=n, we'll get an unused-var
warning for `ten_thousand'. Another sysctl.c wart. I'll fix it.

> {
> .ctl_name = KERN_NGROUPS_MAX,
> --- linux-2.6.orig/include/linux/sysctl.h 2009-04-09 16:23:01.000000000 +0800
> +++ linux-2.6/include/linux/sysctl.h 2009-06-16 12:30:14.000000000 +0800
> @@ -163,6 +163,7 @@ enum
> KERN_MAX_LOCK_DEPTH=74,
> KERN_NMI_WATCHDOG=75, /* int: enable/disable nmi watchdog */
> KERN_PANIC_ON_NMI=76, /* int: whether we will panic on an unrecovered */
> + KERN_PRINTK_DELAY = 77, /* int: tune printk delay*/

No, we don't do this any more. I'll fix that too.

Please retest the result.


diff -puN include/linux/sysctl.h~printk-add-printk_delay-to-make-messages-readable-for-some-scenarios-fix include/linux/sysctl.h
--- a/include/linux/sysctl.h~printk-add-printk_delay-to-make-messages-readable-for-some-scenarios-fix
+++ a/include/linux/sysctl.h
@@ -163,7 +163,6 @@ enum
KERN_MAX_LOCK_DEPTH=74,
KERN_NMI_WATCHDOG=75, /* int: enable/disable nmi watchdog */
KERN_PANIC_ON_NMI=76, /* int: whether we will panic on an unrecovered */
- KERN_PRINTK_DELAY = 77, /* int: tune printk delay*/
};


--- a/kernel/sysctl.c~printk-add-printk_delay-to-make-messages-readable-for-some-scenarios-fix
+++ a/kernel/sysctl.c
@@ -103,7 +103,9 @@ static int __maybe_unused one = 1;
static int __maybe_unused two = 2;
static unsigned long one_ul = 1;
static int one_hundred = 100;
+#ifdef CONFIG_PRINTK
static int ten_thousand = 10000;
+#endif

/* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
static unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
@@ -710,7 +712,7 @@ static struct ctl_table kern_table[] = {
.proc_handler = &proc_dointvec,
},
{
- .ctl_name = KERN_PRINTK_DELAY,
+ .ctl_name = CTL_UNNUMBERED,
.procname = "printk_delay",
.data = &printk_delay_msec,
.maxlen = sizeof(int),
_


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