[dropped all these bouncing email lists. Adding closed lists to public
cc lists is just a bad idea]
> int
> main(int argc, char **argv)
> {
> int ctx_fd;
> pfarg_pmd_t pd[1];
> pfarg_pmc_t pc[1];
> pfarg_ctx_t ctx;
> pfarg_load_t load_args;
>
> memset(&ctx, 0, sizeof(ctx));
> memset(pc, 0, sizeof(pc));
> memset(pd, 0, sizeof(pd));
>
> /* create session (context) and get file descriptor back (identifier) */
> ctx_fd = pfm_create_context(&ctx, NULL, NULL, 0);
There's nothing in your example that makes the file descriptor needed.
>
> /* setup one config register (PMC0) */
> pc[0].reg_num = 0
> pc[0].reg_value = 0x1234;
That would be nicer if it was just two arguments.
>
> /* setup one data register (PMD0) */
> pd[0].reg_num = 0;
> pd[0].reg_value = 0;
Why do you need to set the data register? Wouldn't it make
more sense to let the kernel handle that and just return one.
>
> /* program the registers */
> pfm_write_pmcs(ctx_fd, pc, 1);
> pfm_write_pmds(ctx_fd, pd, 1);
>
> /* attach the context to self */
> load_args.load_pid = getpid();
> pfm_load_context(ctx_fd, &load_args);
My replacement would be to just add a flags argument to write_pmcs
with one flag bit meaning "GLOBAL CONTEXT" versus "MY CONTEXT"
>
> /* activate monitoring */
> pfm_start(ctx_fd, NULL);
Why can't that be done by the call setting up the register?
Or if someone needs to do it for a specific region they can read
the register before and then afterwards.
>
> /*
> * run code to measure
> */
>
> /* stop monitoring */
> pfm_stop(ctx_fd);
>
> /* read data register */
> pfm_read_pmds(ctx_fd, pd, 1);
On x86 i think it would be much simpler to just let the set/alloc
register call return a number and then use RDPMC directly. That would
be actually faster and be much simpler too.
I suppose most architectures have similar facilities, if not a call could be
added for them but it's not really essential. The call might be also needed
for event multiplexing, but frankly I would just leave that out for now.
e.g. here is one use case I would personally see as useful. We need
a replacement for simple cycle counting since RDTSC doesn't do that anymore
on modern x86 CPUs. It could be something like:
/* 0 is the initial value */
/* could be either library or syscall */
event = get_event(COUNTER_CYCLES);
if (event < 0)
/* CPU has no cycle counter */
reg = setup_perfctr(event, 0 /* value */, LOCAL_EVENT); /* syscall */
rdpmc(reg, start);
.... some code to run ...
rdpmc(reg, end);
free_perfctr(reg); /* syscall */
On other architectures rdpmc would be different of course, but
the rest could be probably similar.
-Andi
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