Re: Proposed systemtap access to perfmon hardware

[William Cohen <wcohen@redhat.com>]

To try to get a feel on how the performance monitoring hardware support 
would work in SystemTap I wrote some simple examples. Below are examples 
for computing IPC, average cycle count, and sampling within a function. 
The IPC and average cycle count function need a bit of rework to work 
for a SMP machines.

Let me know if there are comments or questions on the examples.

-Will

COMPUTING IPC

global cycles_h
global instr_retired_h
probe perfmon.event("cycles") {cycles_h = $handle;}
probe perfmon.event("intr_retired") {instr_retired_h = $handle;}

probe begin {print ("start probe");}
probe end
{
	factor=100;
	ipc = (factor*perfmon_get_counter(intr_retired_h))/
	    perfmon_get_counter(cycles_h);
	print ("ipc is %d.%d \n", ipc/factor, ipc % factor);
}




DETERMINING AVERAGE CYCLE COUNT FOR FUNCTION (AND CHILDREN)

global cycles_h

probe perfmon.event("cycles") {
       cycles_h = $handle;
       perfmon_stop_counter(cycles_h);
}

global count

probe kernel.function("blah"){
       ++count;
       perfmon_start_counter(cycles_h);
}

probe kernel.function.return("blah"){
       perfmon_stop_counter(cycles_h);
}

probe begin {print ("start probe");}
probe end
{
	total_cycles=perfmon_stop_counter(cycles_h);
	print ("average count in blah %d\n", total_cycles/count);
}


SAMPLING WITHIN A FUNCTION (AND CHILDREN)

global cycles_h
global where_am_i

probe perfmon.event("cycles").sample(100000) {
       cycles_h = $handle;
       # record where sample occured
       where_am_i[instruction_pointer()]++;
}

global count

probe kernel.function("blah"){
       ++count;
       perfmon_start_counter(cycles_h);
}

probe kernel.function("blah").return{
       perfmon_stop_counter(cycles_h);
}

probe begin
{
       # turn off the sampling
       perfmon_stop_counter(cycles_h);
       print("start probe");
}

probe end
{
	#write out the where_am_i entries
	print("address\tcount\n");
	foreach ([+ip] in where_am_i) {
		print("0x%x\t%d\n", ip, where_am_i[ip]);
	}
}