"Sysrq-p is pretty useless unless you can force the keyboard interrupt and the spinning process onto the same CPU," noted Chuck Ebbert during a discussion centered around debugging tasks stuck in a running state. Pressing the
<Alt><SysRq><p> key combination is used for debugging, dumping the registers and flags from the CPU that handles the keypress interrupt to the console. UltraSPARC maintainer, David Miller, replied, "yes, I find this a painful limitation too," adding:
"Sparc64 used to dump the registers on all active cpus for show_regs() via a cross-call, and this was incredibly useful. But I disabled that as soon as I started playing with Niagara because at 32 cpus and larger the output is just too voluminous to be useful."
David then suggested, "what might be appropriate is just to get a one-line program counter dump on every cpu via some new sysrq keystroke." Chuck noted that similar functionality is provided by a patch in the -mm kernel, "IIRC -mm had something like this but it was buggy because we were sending IPIs to each processor asking them to print their state. Maybe it would work if we had a way of making them dump their state to a memory location and then collected and printed it from the CPU that's handling the sysrq."