linux-kernel - Re: [perfmon] Re: [perfmon2] perfmon2 merge news

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Message-ID: <20071114020702.GB20365@one.firstfloor.org>
Date:	Wed, 14 Nov 2007 03:07:02 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Stephane Eranian <eranian@....hp.com>
Cc:	Andi Kleen <andi@...stfloor.org>, akpm@...l.org,
	Robert Richter <robert.richter@....com>, gregkh@...e.de,
	linux-kernel@...r.kernel.org, William Cohen <wcohen@...hat.com>
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news

[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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