Em Sat, Sep 22, 2007 at 02:22:00PM -0400, Cliff Brake escreveu: > I have a system where I am doing the following on a PXA270 ARM system: > > An applications is sending and receiving a packet on a serial port > every 40ms. With 2.6.20, I simply enabled kernel preemption and set > the priority of my application to real-time, and it worked great, no > matter what the system load. You didn't made it clear what exactly is the kernel version you are using. You mention 2.6.20, but is this with or without the PREEMPT_RT patch? > With 2.6.23-rc6, I have enabled CONFIG_HIGH_RES_TIMERS & > CONFIG_PREEMPT. Scheduling in the real-time application is rock solid > and looking at timing with a scope, and instrumenting the loop in the > application indicates the 40ms is very solid. However, it seems the > receive data is getting stuck in the serial driver for relatively long > periods of time (>= 40ms), which is a problem in this system. > > Is there anything that would have changed between 2.6.20 and > 2.6.23-rc6 that would explain this? What priority do pieces of the > serial driver run at? Is there any way to make the serial driver run > at high priority so that data is processed in a timely manner? Look at the IRQ-<N> kernel thread that services the serial port hardware interrupt, use chrt to bump its priority and see if it helps. To see which is the interrupt: [root@tonchinha ~]# dmesg | grep ttyS0 Kernel command line: root=/dev/hda1 console=ttyS0,115200 console=tty0 ignore_loglevel serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A 00:02: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A Now check what is the default rt priority for its hard interrupt kernel thread handler, "IRQ-4": [root@tonchinha ~]# ps -C IRQ-4 -To pid,rtprio,cmd PID RTPRIO CMD 1721 50 [IRQ-4] [root@tonchinha ~]# Use the `chrt' utility to bump the priority to, say, 75: [root@tonchinha ~]# chrt -p 75 1721 Check again: [root@tonchinha ~]# ps -C IRQ-4 -To pid,rtprio,cmd PID RTPRIO CMD 1721 75 [IRQ-4] - Arnaldo - To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
