Re: [perfmon] Re: [perfmon2] perfmon2 merge news

[Stephane Eranian <eranian@hpl.hp.com>]

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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