On Mon, 2012-04-23 at 13:07 -0400, Brian J. Murrell wrote: > On 12-04-22 04:56 PM, Andy Walls wrote: > > > > If, in your system, IRQ service for device A under some circumstances > > has precendence over IRQ service for the CX23418 and hence holds off its > > service; and the irq handler in the driver for device A decides to > > perform some some long I/O operations with device A; then it doesn't > > matter how fast your CPU is. > > Yes, quite true. I was forgetting about how nasty an irq handler can be > on other hardware. > > > You may wish to use perf or ftrace, or some other tool/method of > > measuring kernel interrupt handling latency to find out what causes any > > delays from the CX23418 raising its IRQ line to cx18_irq_handler() being > > called by the kernel. > > Excellent idea. I'm afraid I'm quite (read: very) green in the area of > kernel performance profiling. Here's an example of me checking latencies in the ivtv driver: http://www.spinics.net/lists/linux-media/msg15762.html Here are some good articles: http://lwn.net/Articles/322666/ http://lwn.net/Articles/322731/ http://lwn.net/Articles/366796/ http://lwn.net/Articles/365835/ http://lwn.net/Articles/410200/ http://lwn.net/Articles/425583/ http://lwn.net/Articles/370423/ http://people.redhat.com/srostedt/ftrace-tutorial.odp https://events.linuxfoundation.org/slides/2010/linuxcon_japan/linuxcon_jp2010_rostedt.pdf An account of an in depth investigation into the maximum interrupts disabled duration on some Sony of America test systems: https://events.linuxfoundation.org/slides/2010/linuxcon_japan/linuxcon_jp2010_rowand.pdf > But I'm smart. Looking around, it seems > that with ftrace, I am looking for the irqsoff tracer, is that correct? That sounds good as good as any. (I always just end up learning what I have to do when I need it.) > Unfortunately my kernel doesn't have that one: It was probably just compiled without it. On my Fedora 15 system, that is the case with the stock kernel: [andy@palomino ~]$ grep IRQSOFF_TRACER /boot/config-`uname -r` # CONFIG_IRQSOFF_TRACER is not set > # cat /sys/kernel/debug/tracing/available_tracers > blk function_graph mmiotrace wakeup_rt wakeup function nop *sigh* So much for being convenient and easy. > I can't seem to find any useful information on using perf to analyze ISR > latency. Any pointers? Not really. I have never used perf. I assume you found this page: https://perf.wiki.kernel.org/ > Cheers, > b. Regards, Andy -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
