linux-kernel - Re: [PATCH 5/6] perf_counter: add more context information

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Message-Id: <1239015668.798.4243.camel@twins>
Date:	Mon, 06 Apr 2009 13:01:08 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] perf_counter: add more context information

On Fri, 2009-04-03 at 11:25 -0700, Corey Ashford wrote:
> Peter Zijlstra wrote:
> > On Thu, 2009-04-02 at 11:12 +0200, Peter Zijlstra wrote:
> >> plain text document attachment (perf_counter_callchain_context.patch)
> >> Put in counts to tell which ips belong to what context.
> >>
> >>   -----
> >>    | |  hv
> >>    | --
> >> nr | |  kernel
> >>    | --
> >>    | |  user
> >>   -----
> > 
> > Right, just realized that PERF_RECORD_IP needs something similar if one
> > if not able to derive the context from the IP itself..
> > 
> Three individual bits would suffice, or you could use a two-bit code -
> 00 = user
> 01 = kernel
> 10 = hypervisor
> 11 = reserved (or perhaps unknown)
> 
> Unfortunately, because of alignment, it would need to take up another 64 
> bit word, wouldn't it?  Too bad you cannot sneak the bits into the IP in 
> a machine independent way.
> 
> And since you probably need a separate word, that effectively doubles 
> the amount of space taken up by IP samples (if we add a "no event 
> header" option).  Should we add another bit in the record_type field - 
> PERF_RECORD_IP_LEVEL (or similar) so that user-space apps don't have to 
> get this if they don't need it?

If we limit the event size to 64k (surely enough, right? :-), then we
have 16 more bits to play with in the header, and we could do something
like the below.

A further possibility would also be to add an overflow bit in there,
making the full 32bit PERF_RECORD space available to output events as
well.

Index: linux-2.6/include/linux/perf_counter.h
===================================================================
--- linux-2.6.orig/include/linux/perf_counter.h
+++ linux-2.6/include/linux/perf_counter.h
@@ -201,9 +201,17 @@ struct perf_counter_mmap_page {
 	__u32   data_head;		/* head in the data section */
 };
 
+enum {
+	PERF_EVENT_LEVEL_HV	= 0,
+	PERF_EVENT_LEVEL_KERNEL = 1,
+	PERF_EVENT_LEVEL_USER	= 2,
+};
+
 struct perf_event_header {
 	__u32	type;
-	__u32	size;
+	__u16	level		:  2,
+		__reserved	: 14;
+	__u16	size;
 };
 
 enum perf_event_type {
Index: linux-2.6/kernel/perf_counter.c
===================================================================
--- linux-2.6.orig/kernel/perf_counter.c
+++ linux-2.6/kernel/perf_counter.c
@@ -1832,6 +1832,8 @@ static void perf_counter_output(struct p
 
 	header.type = PERF_EVENT_COUNTER_OVERFLOW;
 	header.size = sizeof(header);
+	header.level = user_mode(regs) ? 
+		PERF_EVENT_LEVEL_USER : PERF_EVENT_LEVEL_KERNEL;
 
 	if (record_type & PERF_RECORD_IP) {
 		ip = instruction_pointer(regs);


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