From: Milian Wolff <milian.wolff@kdab.com>
To: Milian Wolff <milian.wolff@kdab.com>
Cc: Andi Kleen <ak@linux.intel.com>, Jiri Olsa <jolsa@redhat.com>,
linux-kernel@vger.kernel.org, Jiri Olsa <jolsa@kernel.org>,
namhyung@kernel.org, linux-perf-users@vger.kernel.org,
Arnaldo Carvalho <acme@kernel.org>
Subject: Re: PEBS level 2/3 breaks dwarf unwinding! [WAS: Re: Broken dwarf unwinding - wrong stack pointer register value?]
Date: Thu, 08 Nov 2018 13:41:50 +0100 [thread overview]
Message-ID: <7105958.qy8IzDpmRc@agathebauer> (raw)
In-Reply-To: <1584560.aXQ729uQp3@agathebauer>
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On Mittwoch, 7. November 2018 23:41:31 CET Milian Wolff wrote:
> On Dienstag, 6. November 2018 21:24:11 CET Andi Kleen wrote:
> > > Where would I look for the source to change here? So far, I only
> > > concentrated on the userspace side of perf in tools/perf.
> >
> > Kind of similar to
> >
> > a405bad5ad20 perf/x86: Add Haswell specific transaction flag reporting
> > fdfbbd07e91f perf: Add generic transaction flags
> >
> > Report the original (not overwritten) regs->ip and regs->sp
>
> Thanks a lot Andi! With your help, I have managed to find the exact issue
> for my scenario. Turns out, it really is "just" the instruction pointer
> that is wrong. I.e. originally we have IP = 0x7feda32ca68c, but with PEBS
> we correct that to IP = 7feda32ca688. The SP register value stays the same
> according to my printk output. Using the original IP value, we can unwind
> correctly since we point to the correct place in the .eh_frame section. The
> PEBS IP points to a different position in the .eh_frame section, which is
> "too early".
>
> That brings up some questions:
>
> - I noticed `perf record --intr-regs`, but the values recorded in the
> perf.data file are always the same. I.e. comparing uregs and iregs, I always
> see the same values printed by `perf script`. This smells like a bug to me,
> but so far I haven't figured out why this happens...
The reason seems to be that perf_event_output only takes one set of registers,
which then gets handed down into perf_prepare_sample where it gets sampled.
Thus if sample type has both PERF_SAMPLE_REGS_USER and PERF_SAMPLE_REGS_INTR
set, then by design both will store the same values for user space samples.
Can we change this, such that perf_event_output also takes a second set of
registers (iregs) that get sampled for PERF_SAMPLE_REGS_INTR? I'm very new to
real kernel development, what kind of ABI/API stability guarantees exist for
something like "perf_event_output"?
> - Independently, when I add a custom printk manually in `arch/x86/events/
> intel/ds.c` at the end of `setup_pebs_sample_data`, then I'm never seeing
> any differences between SP in iregs/pebs/regs. Shouldn't it also be
> recorded via PEBS? Or is it just chance that I'm never seeing any
> difference in setup_pebs_sample_data between iregs->sp and regs->sp?
The reason here seems to be that the registers stored in "pebs" are
essentially the same as iregs for the setup for `perf record --call-graph
dwarf`. The difference is the availability of `pebs->real_ip` which gets used
on my system to fixup the IP. SP stays untouched and is thus only truly valid
for the untouched IP (which is discarded currently - see above).
> - Generally, how do we want to handle this bug? If `--intr-regs` would
> actually record a different IP than stored in uregs in the perf.data file,
> then we could use that as a fallback for unwinding, when it fails the first
> time. Or should we always unwind from that IP? How do we mark the "actual"
> frame/IP then, if that differs?
>
> Thanks
--
Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts
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next prev parent reply other threads:[~2018-11-08 12:42 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-20 22:39 Broken dwarf unwinding - wrong stack pointer register value? Milian Wolff
2018-10-21 20:32 ` Milian Wolff
2018-10-22 10:35 ` Milian Wolff
2018-10-22 11:17 ` Milian Wolff
2018-10-22 13:58 ` Andi Kleen
2018-10-22 19:26 ` Milian Wolff
2018-10-23 4:03 ` Andi Kleen
2018-10-23 10:34 ` Milian Wolff
2018-10-24 14:48 ` Andi Kleen
2018-10-30 22:34 ` Milian Wolff
2018-11-01 22:08 ` PEBS level 2/3 breaks dwarf unwinding! [WAS: Re: Broken dwarf unwinding - wrong stack pointer register value?] Milian Wolff
2018-11-02 11:26 ` Jiri Olsa
2018-11-02 17:56 ` Milian Wolff
2018-11-05 20:51 ` Jiri Olsa
2018-11-05 22:54 ` Milian Wolff
2018-11-06 0:10 ` Andi Kleen
2018-11-06 8:39 ` Jiri Olsa
2018-11-06 17:26 ` Andi Kleen
2018-11-06 20:04 ` Milian Wolff
2018-11-06 20:24 ` Andi Kleen
2018-11-07 22:41 ` Milian Wolff
2018-11-08 12:41 ` Milian Wolff [this message]
2018-11-09 0:55 ` Andi Kleen
2018-11-09 0:54 ` Andi Kleen
2018-11-10 21:42 ` Travis Downs
2018-11-11 1:07 ` Andi Kleen
[not found] ` <CAOBGo4zirLiKX8VcROAE=kAD0+qkF0E-cBv9DtBiQr=_obDv5w@mail.gmail.com>
2018-11-11 2:54 ` Travis Downs
2018-11-12 3:26 ` Andi Kleen
2018-11-14 13:20 ` Milian Wolff
2018-11-15 2:05 ` Travis Downs
2018-11-15 9:10 ` Milian Wolff
2018-11-15 19:00 ` Andi Kleen
2018-11-15 2:15 ` Travis Downs
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