Re: [PATCH] serial: core: prevent softlockups on slow consoles
From: Peter Hurley
Date: Wed Sep 02 2015 - 10:38:33 EST
On 08/31/2015 10:34 AM, Vitaly Kuznetsov wrote:
> Hyper-V serial port is very slow on multi-vCPU guest
How slow and why?
> this causes
> soflockups on intensive console writes. Touch nmi watchdog after putting
> every char on port to avoid the issue for all serial drivers, the overhead
> should be small.
Once per message should be sufficient.
Although I have no objection to adding this to uart_console_write() for
all serial drivers, please remove it from drivers that already do this.
> This is just a part of the fix: serial8250_console_write() disables irqs
> for all its execution time
Interrupts are disabled by printk()/console_unlock() right now anyway;
very thorny problem to fix with lots of complications.
> (which on such slow consoles can be dozens of
> seconds), it should be possible to observe devices being stuck on this
> CPU. We need to find a better way, e.g. do output in batches enabling irqs
> in between.
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> drivers/tty/serial/serial_core.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index f368520..cc05785 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -33,7 +33,7 @@
> #include <linux/serial.h> /* for serial_state and serial_icounter_struct */
> #include <linux/serial_core.h>
> #include <linux/delay.h>
> -#include <linux/mutex.h>
Why isn't this required anymore?
> +#include <linux/nmi.h>
> #include <asm/irq.h>
> #include <asm/uaccess.h>
> @@ -1792,6 +1792,7 @@ void uart_console_write(struct uart_port *port, const char *s,
> if (*s == '\n')
> putchar(port, '\r');
> putchar(port, *s);
> + touch_nmi_watchdog();
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