Re: [PATCH] serial: core: prevent softlockups on slow consoles
From: Peter Hurley <hidden>
Date: 2015-09-02 14:38:30
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Hi Vitaly, 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. Regards, Peter Hurley
quoted hunk ↗ jump to hunk
(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@redhat.com> --- 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?
quoted hunk ↗ jump to hunk
+#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(); } } EXPORT_SYMBOL_GPL(uart_console_write);