Re: [PATCH v2 0/6] perf: Introduce extended syscall error reporting

Alexander Shishkin
2015-08-26T16:59:01Z

Ingo Molnar <mingo@kernel.org> writes:

> * Ingo Molnar <mingo@kernel.org> wrote:
>
>> ... but back then I didn't feel like complicating an error recovery ABI for the
>> needs of the 1%, robust error handling is all about simplicity: if it's not
>> simple, tools won't use it.
>
> And note that it needs to be 'simple' in two places for usage to grow naturally:
>
> - the usage site in the kernel
> - the tooling side that recovers the information.
>
> That's why I think that such a form:
>
> return err_str(-EINVAL, "x86/perf: CPU does not support precise sampling");
>
> is obviously simple on the kernel side as it returns -EINVAL, and is very simple
> on the tooling side as well, if we are allowed to extend prctl().

So I hacked stuff a bit [1] to accomodate some of the above
ideas. The below diff shows how these ideas integrate with perf. The
rest is in my github tree.

- this exterr implementation allows its users to add arbitrary
information to the call site structures and also pretty print them on
the way out; in the example, perf stores perf_event_attr field name
that is the source of trouble; it's a string rather than offsetof(),
because half of our event attribute is a bit field;
- the "way out" doesn't have to be syscall return path (although in the
example it is);
- userspace can fetch the extended error reports via prctl() like you
suggested above;
- error codes are still passed around in the [-EXT_ERRNO..-MAX_ERRNO]
range until they are passed to userspace (which is where ext_err_code()
converts them back to traditional errno.h values).

# perf record -e branches -c1 ls
kernel says (0/95): {
"file": "/home/ash/work/linux/arch/x86/kernel/cpu/perf_event.c",
"line": 432,
"code": -95,
"module": "perf/x86",
"message": "BTS sampling not allowed for kernel space"
, "attr_field": "exclude_kernel"
}

Error:
No hardware sampling interrupt available.
No APIC? If so then you can boot the kernel with the "lapic" boot parameter to force-enable it.
#

[1] https://github.com/virtuoso/linux-perf/commits/exterr

>From 1338fe417223cc1d8bc7588d95c6a913993d966f Mon Sep 17 00:00:00 2001
From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Date: Wed, 26 Aug 2015 18:36:14 +0300
Subject: [PATCH] perf: Use extended syscall error reporting somewhat

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
---
arch/x86/kernel/cpu/perf_event.c | 5 ++++-
include/linux/perf_event.h | 14 ++++++++++++++
kernel/events/core.c | 17 ++++++++++++++++-
3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index f56cf074d0..5e8f2edb2c 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -12,6 +12,8 @@
* For licencing details see kernel-base/COPYING
*/

+#define EXTERR_MODNAME "perf/x86"
+
#include <linux/perf_event.h>
#include <linux/capability.h>
#include <linux/notifier.h>
@@ -426,7 +428,8 @@ int x86_setup_perfctr(struct perf_event *event)

/* BTS is currently only allowed for user-mode. */
if (!attr->exclude_kernel)
- return -EOPNOTSUPP;
+ return perf_err(-EOPNOTSUPP, exclude_kernel,
+ "BTS sampling not allowed for kernel space");

/* disallow bts if conflicting events are present */
if (x86_add_exclusive(x86_lbr_exclusive_lbr))
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 2027809433..eb63074012 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -16,6 +16,20 @@

#include <uapi/linux/perf_event.h>

+#include <linux/exterr.h>
+
+struct perf_ext_err_site {
+ struct ext_err_site site;
+ const char *attr_field;
+};
+
+#define perf_err(__c, __a, __m) \
+ ({ /* make sure it's a real field before stringifying it */ \
+ struct perf_event_attr __x; (void)__x.__a; \
+ ext_err(perf, __c, __m, \
+ .attr_field = __stringify(__a)); \
+ })
+
/*
* Kernel-internal data types and definitions:
*/
diff --git a/kernel/events/core.c b/kernel/events/core.c
index ae16867670..5523c623c4 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9,6 +9,8 @@
* For licensing details see kernel-base/COPYING
*/

+#define EXTERR_MODNAME "perf"
+
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/cpu.h>
@@ -44,11 +46,24 @@
#include <linux/compat.h>
#include <linux/bpf.h>
#include <linux/filter.h>
+#include <linux/exterr.h>

#include "internal.h"

#include <asm/irq_regs.h>

+static char *perf_exterr_format(void *site)
+{
+ struct perf_ext_err_site *psite = site;
+ char *output;
+
+ output = kasprintf(GFP_KERNEL, ",\t\"attr_field\": \"%s\"\n",
+ psite->attr_field);
+ return output;
+}
+
+DECLARE_EXTERR_DOMAIN(perf, perf_exterr_format);
+
static struct workqueue_struct *perf_wq;

typedef int (*remote_function_f)(void *);
@@ -8352,7 +8367,7 @@ err_group_fd:
fdput(group);
err_fd:
put_unused_fd(event_fd);
- return err;
+ return ext_err_errno(err);
}

/**
--
2.5.0