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Date: Tue, 11 Oct 2022 11:03:39 +0200
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Petr Mladek <pmladek@...e.com>,
Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Do not delay boot when printing log to serial console during startup?
Dear Linux folks,
We boot our systems generally with the serial console enabled
`console=ttyS0,115200n8`, and noticed that initializing the console
takes at least one second.
$ dmesg | grep -e 'DMI:' -e 'printk: console'
[ 0.000000] DMI: Dell Inc. PowerEdge R7525/05Y13N, BIOS 2.7.3
03/30/2022
[ 2.691432] printk: console [tty0] enabled
[ 5.414384] printk: console [ttyS0] enabled
$ dmesg | grep -e 'DMI:' -e 'printk:'
[ 0.000000] DMI: Dell Inc. Precision Tower 3620/0MWYPT, BIOS
2.22.0 07/13/2022
[ 0.146953] printk: console [tty0] enabled
[ 1.374382] printk: console [ttyS0] enabled
$ dmesg | grep -e 'DMI:' -e 'printk: console'
[ 0.000000] DMI: Dell Inc. PowerEdge R7425/08V001, BIOS 1.6.7
10/29/2018
[ 1.589543] printk: console [tty0] enabled
[ 3.057770] printk: console [ttyS0] enabled
$ dmesg | grep -e 'DMI:' -e 'printk: console'
[ 0.000000] DMI: HPE ProLiant DL385 Gen10 Plus/ProLiant DL385
Gen10 Plus, BIOS A42 12/03/2021
[ 5.171202] printk: console [tty0] enabled
[ 8.066602] printk: console [ttyS0] enabled
`initcall_debug` shows:
[ 0.190491] calling univ8250_console_init+0x0/0x2b @ 0
[ 1.488645] printk: console [ttyS0] enabled
[ 1.492945] initcall univ8250_console_init+0x0/0x2b returned 0
after 0 usecs
(Note, that the “after time” by initcall does not match the timestamp at
the beginning.)
Tracing with the trace *function* (`ftrace=function`) shows:
<idle>-0 [000] ...2. 0.213187:
univ8250_console_init <-console_init
<idle>-0 [000] ...2. 0.213187:
serial8250_isa_init_ports <-univ8250_console_init
<idle>-0 [000] ...2. 0.213187: serial8250_init_port
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213187: init_timer_key
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213187:
serial8250_set_defaults <-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188: serial8250_init_port
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188: init_timer_key
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188:
serial8250_set_defaults <-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188: serial8250_init_port
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188: init_timer_key
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188:
serial8250_set_defaults <-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188: serial8250_init_port
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188: init_timer_key
<-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213188:
serial8250_set_defaults <-serial8250_isa_init_ports
<idle>-0 [000] ...2. 0.213189: register_console
<-univ8250_console_init
<idle>-0 [000] ...2. 0.213189:
try_enable_preferred_console <-register_console
<idle>-0 [000] ...2. 0.213189:
univ8250_console_match <-try_enable_preferred_console
<idle>-0 [000] ...2. 0.213189:
univ8250_console_setup <-try_enable_preferred_console
<idle>-0 [000] ...2. 0.213189:
serial8250_console_setup <-univ8250_console_setup
<idle>-0 [000] ...2. 0.213189: uart_parse_options
<-serial8250_console_setup
<idle>-0 [000] ...2. 0.213190: uart_set_options
<-serial8250_console_setup
<idle>-0 [000] ...2. 0.213190:
tty_termios_encode_baud_rate <-uart_set_options
<idle>-0 [000] ...2. 0.213190:
serial8250_set_termios <-uart_set_options
<idle>-0 [000] ...2. 0.213191:
serial8250_do_set_termios <-uart_set_options
<idle>-0 [000] ...2. 0.213191: tty_get_char_size
<-serial8250_do_set_termios
<idle>-0 [000] ...2. 0.213191: uart_get_baud_rate
<-serial8250_do_set_termios
<idle>-0 [000] ...2. 0.213191:
tty_termios_baud_rate <-uart_get_baud_rate
<idle>-0 [000] ...2. 0.213191:
serial8250_get_divisor <-serial8250_do_set_termios
<idle>-0 [000] ...2. 0.213191: uart_get_divisor
<-serial8250_get_divisor
<idle>-0 [000] ...2. 0.213191:
_raw_spin_lock_irqsave <-serial8250_do_set_termios
<idle>-0 [000] d..2. 0.213192: preempt_count_add
<-_raw_spin_lock_irqsave
<idle>-0 [000] d..3. 0.213192: uart_update_timeout
<-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213192: tty_get_frame_size
<-uart_update_timeout
<idle>-0 [000] d..3. 0.213192: io_serial_out
<-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213195:
serial8250_do_set_divisor <-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213195: io_serial_out
<-serial8250_do_set_divisor
<idle>-0 [000] d..3. 0.213198:
default_serial_dl_write <-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213198: io_serial_out
<-default_serial_dl_write
<idle>-0 [000] d..3. 0.213201: io_serial_out
<-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213204: io_serial_out
<-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213207: io_serial_out
<-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213210:
serial8250_do_set_mctrl <-serial8250_do_set_termios
<idle>-0 [000] d..3. 0.213210: io_serial_out
<-serial8250_do_set_mctrl
<idle>-0 [000] d..3. 0.213213:
_raw_spin_unlock_irqrestore <-serial8250_do_set_termios
<idle>-0 [000] ...3. 0.213213: preempt_count_sub
<-_raw_spin_unlock_irqrestore
<idle>-0 [000] ...2. 0.213213:
tty_termios_baud_rate <-serial8250_do_set_termios
<idle>-0 [000] ...2. 0.213214:
tty_termios_encode_baud_rate <-serial8250_do_set_termios
<idle>-0 [000] ...2. 0.213214: console_lock
<-register_console
<idle>-0 [000] ...2. 0.213214: __cond_resched
<-console_lock
<idle>-0 [000] ...2. 0.213214: down <-console_lock
<idle>-0 [000] ...2. 0.213214: __cond_resched <-down
<idle>-0 [000] ...2. 0.213214:
_raw_spin_lock_irqsave <-down
<idle>-0 [000] d..2. 0.213214: preempt_count_add
<-_raw_spin_lock_irqsave
<idle>-0 [000] d..3. 0.213214:
_raw_spin_unlock_irqrestore <-console_lock
<idle>-0 [000] ...3. 0.213214: preempt_count_sub
<-_raw_spin_unlock_irqrestore
<idle>-0 [000] ...2. 0.213214: mutex_lock
<-register_console
<idle>-0 [000] ...2. 0.213215: __cond_resched
<-mutex_lock
<idle>-0 [000] ...2. 0.213215: mutex_unlock
<-register_console
<idle>-0 [000] ...2. 0.213215: console_unlock
<-register_console
<idle>-0 [000] ...2. 0.213215:
console_emit_next_record.constprop.0 <-console_unlock
<idle>-0 [000] ...2. 0.213215: prb_read_valid
<-console_emit_next_record.constprop.0
<idle>-0 [000] ...2. 0.213215: _prb_read_valid
<-prb_read_valid
<idle>-0 [000] ...2. 0.213215:
desc_read_finalized_seq <-_prb_read_valid
<idle>-0 [000] ...2. 0.213215:
console_emit_next_record.constprop.0 <-console_unlock
<idle>-0 [000] ...2. 0.213215: prb_read_valid
<-console_emit_next_record.constprop.0
<idle>-0 [000] ...2. 0.213215: _prb_read_valid
<-prb_read_valid
<idle>-0 [000] ...2. 0.213215:
desc_read_finalized_seq <-_prb_read_valid
<idle>-0 [000] ...2. 0.213216: get_data
<-_prb_read_valid
<idle>-0 [000] ...2. 0.213216:
desc_read_finalized_seq <-_prb_read_valid
<idle>-0 [000] ...2. 0.213216: record_print_text
<-console_emit_next_record.constprop.0
<idle>-0 [000] ...2. 0.213216: info_print_prefix
<-record_print_text
<idle>-0 [000] d..2. 0.213216: __printk_safe_enter
<-console_emit_next_record.constprop.0
<idle>-0 [000] d..2. 0.213216: _raw_spin_lock
<-console_emit_next_record.constprop.0
<idle>-0 [000] d..2. 0.213216: preempt_count_add
<-_raw_spin_lock
<idle>-0 [000] d..3. 0.213216: _raw_spin_unlock
<-console_emit_next_record.constprop.0
<idle>-0 [000] d..3. 0.213217: preempt_count_sub
<-_raw_spin_unlock
<idle>-0 [000] d..2. 0.213217:
univ8250_console_write <-console_emit_next_record.constprop.0
<idle>-0 [000] d..2. 0.213217:
serial8250_console_write <-console_emit_next_record.constprop.0
<idle>-0 [000] d..2. 0.213217:
_raw_spin_lock_irqsave <-serial8250_console_write
<idle>-0 [000] d..2. 0.213217: preempt_count_add
<-_raw_spin_lock_irqsave
<idle>-0 [000] d..3. 0.213217: io_serial_in
<-serial8250_console_write
<idle>-0 [000] d..3. 0.213220: io_serial_out
<-serial8250_console_write
<idle>-0 [000] d..3. 0.213223: uart_console_write
<-serial8250_console_write
<idle>-0 [000] d..3. 0.213223:
serial8250_console_putchar <-uart_console_write
<idle>-0 [000] d..3. 0.213223: wait_for_xmitr
<-serial8250_console_putchar
<idle>-0 [000] d..3. 0.213223: io_serial_in
<-wait_for_xmitr
<idle>-0 [000] d..3. 0.213226: io_serial_out
<-uart_console_write
<idle>-0 [000] d..3. 0.213229:
serial8250_console_putchar <-uart_console_write
<idle>-0 [000] d..3. 0.213229: wait_for_xmitr
<-serial8250_console_putchar
<idle>-0 [000] d..3. 0.213229: io_serial_in
<-wait_for_xmitr
<idle>-0 [000] d..3. 0.213232: __const_udelay
<-wait_for_xmitr
<idle>-0 [000] d..3. 0.213232: delay_tsc
<-wait_for_xmitr
<idle>-0 [000] d..3. 0.213233: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213233: preempt_count_sub
<-delay_tsc
<idle>-0 [000] d..3. 0.213233: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213233: preempt_count_sub
<-delay_tsc
<idle>-0 [000] d..3. 0.213233: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213233: preempt_count_sub
<-delay_tsc
<idle>-0 [000] d..3. 0.213233: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213233: preempt_count_sub
<-delay_tsc
<idle>-0 [000] d..3. 0.213233: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213233: preempt_count_sub
<-delay_tsc
<idle>-0 [000] d..3. 0.213234: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213234: preempt_count_sub
<-delay_tsc
<idle>-0 [000] d..3. 0.213234: io_serial_in
<-wait_for_xmitr
<idle>-0 [000] d..3. 0.213237: __const_udelay
<-wait_for_xmitr
<idle>-0 [000] d..3. 0.213237: delay_tsc
<-wait_for_xmitr
<idle>-0 [000] d..3. 0.213237: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213237: preempt_count_sub
<-delay_tsc
<idle>-0 [000] d..3. 0.213237: preempt_count_add
<-delay_tsc
<idle>-0 [000] d..4. 0.213237: preempt_count_sub
<-delay_tsc
So it looks to me like, that the already recorded messages are printed
out over the serial console first, before it continues. The
documentation of `register_console()` in `kernel/printk/printk.c`
confirms that:
> * The console driver calls this routine during kernel initialization
> * to register the console printing procedure with printk() and to
> * print any messages that were printed by the kernel before the
> * console driver was initialized.
Is there an option to enable the serial console but send the messages to
it asynchronously so it does not hold up the boot?
Kind regards,
Paul
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