Messages in this thread | | | Date | Thu, 24 Sep 2020 08:33:52 +0200 | From | Greg KH <> | Subject | Re: [PATCH 2/2] printk: Make the console flush configurable in hotplug path |
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On Wed, Sep 23, 2020 at 05:08:32PM -0700, Prasad Sodagudi wrote: > From: Mohammed Khajapasha <mkhaja@codeaurora.org> > > The thread which initiates the hot plug can get scheduled > out, while trying to acquire the console lock, > thus increasing the hot plug latency. This option > allows to selectively disable the console flush and > in turn reduce the hot plug latency. > > Signed-off-by: Mohammed Khajapasha <mkhaja@codeaurora.org> > Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org> > --- > init/Kconfig | 10 ++++++++++ > kernel/printk/printk.c | 10 ++++++++-- > 2 files changed, 18 insertions(+), 2 deletions(-) > > diff --git a/init/Kconfig b/init/Kconfig > index d6a0b31..9ce39ba 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -699,6 +699,16 @@ config LOG_BUF_SHIFT > 13 => 8 KB > 12 => 4 KB > > +config CONSOLE_FLUSH_ON_HOTPLUG > + bool "Enable console flush configurable in hot plug code path" > + depends on HOTPLUG_CPU > + def_bool n
n is the default, no need to list it.
> + help > + In cpu hot plug path console lock acquire and release causes the > + console to flush. If console lock is not free hot plug latency > + increases. So make console flush configurable in hot plug path > + and default disabled to help in cpu hot plug latencies.
Why would you not want this option?
Why isn't this just a bugfix?
> + > config LOG_CPU_MAX_BUF_SHIFT > int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)" > depends on SMP > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 9b75f6b..f02d3ef 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -2283,6 +2283,8 @@ void resume_console(void) > console_unlock(); > } > > +#ifdef CONFIG_CONSOLE_FLUSH_ON_HOTPLUG > + > /** > * console_cpu_notify - print deferred console messages after CPU hotplug > * @cpu: unused > @@ -2302,6 +2304,8 @@ static int console_cpu_notify(unsigned int cpu) > return 0; > } > > +#endif > + > /** > * console_lock - lock the console system for exclusive use. > * > @@ -2974,7 +2978,7 @@ void __init console_init(void) > static int __init printk_late_init(void) > { > struct console *con; > - int ret; > + int ret = 0; > > for_each_console(con) { > if (!(con->flags & CON_BOOT)) > @@ -2996,13 +3000,15 @@ static int __init printk_late_init(void) > unregister_console(con); > } > } > +#ifdef CONFIG_CONSOLE_FLUSH_ON_HOTPLUG
#ifdef in .c code is a mess to maintain.
> ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL, > console_cpu_notify); > WARN_ON(ret < 0); > ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "printk:online", > console_cpu_notify, NULL); > WARN_ON(ret < 0); > - return 0; > +#endif
What happens if we don't make these calls entirely? Why not just remove them as who wants extra latency for their system?
thanks,
greg k-h
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