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

Arnaldo Carvalho de Melo
2015-08-26T20:58:30Z

Em Wed, Aug 26, 2015 at 07:56:47PM +0300, Alexander Shishkin escreveu:
> 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).

Hey, can we see the builtin-record.c patch please?

- Arnaldo

> # 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