Andi,
On Tue, Nov 13, 2007 at 11:25:34PM +0100, Andi Kleen wrote:
> On Tue, Nov 13, 2007 at 02:22:34PM -0800, Stephane Eranian wrote:
> > Andi,
> > On Tue, Nov 13, 2007 at 10:50:56PM +0100, Andi Kleen wrote:
> > > > Yes, horribly more complicated because of locking issues within perfmon.
> > > > As soon as you expose a file descriptor, you need some locking to prevent
> > > > multiple user threads (malicious or not) to compete to access the PMU state.
> > >
> > > Why do you need the file descriptor?
> > >
> >
> > To identify your monitoring session be it system-wide (i.e., per-cpu) or per-thread.
> > file descriptor allows you to use close, read, select, poll and you leverage the
>
> Surely that could be done with a flag for each call too? Keeping file descriptors
> to pass essentially a boolean seems overkill.
>
I don't understand this.
Let's take the simplest possible example (self-monitoring per-thread)
counting one event in one data register.
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);
/* setup one config register (PMC0) */
pc[0].reg_num = 0
pc[0].reg_value = 0x1234;
/* setup one data register (PMD0) */
pd[0].reg_num = 0;
pd[0].reg_value = 0;
/* 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);
/* activate monitoring */
pfm_start(ctx_fd, NULL);
/*
* run code to measure
*/
/* stop monitoring */
pfm_stop(ctx_fd);
/* read data register */
pfm_read_pmds(ctx_fd, pd, 1);
printf("PMD0 %llu\n", pd[0].reg_value);
/* destroy session */
close(ctx_fd);
return 0;
}
--
-Stephane
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