Re: broken cycle counts from perf record in frequency mode [Was: Re: deducing CPU clock rate over time from cycle samples]

[Andi Kleen]

Adding Peter.

The original thread starts at
https://www.spinics.net/lists/linux-perf-users/msg03486.html

On Mon, Aug 28, 2017 at 04:40:43PM +0200, Milian Wolff wrote:
> On Monday, August 28, 2017 4:08:47 PM CEST Milian Wolff wrote:
> > On Sunday, June 18, 2017 9:53:05 PM CEST Milian Wolff wrote:
> > > On Sonntag, 18. Juni 2017 06:22:19 CEST Andi Kleen wrote:
> > > > Milian Wolff <milian.wolff@xxxxxxxx> writes:
> > > > > But when I look at the naively calculated first derivative, to
> > > > > visualize
> > > > > CPU load, i.e. CPU clock rate in Hz, then things start to become
> > > > > somewhat
> > > > > confusing:
> > > > > 
> > > > > ~~~~
> > > > > perf script -F time,period | awk 'BEGIN {lastTime = -1;} { time = $1 +
> > > > > 0.0; if (lastTime != -1) {printf("%.6f\t%f\n", time, $2 / (time -
> > > > > lastTime));} lastTime = time; }' | gnuplot -p -e "plot '-' with
> > > > > linespoints"
> > > > > ~~~~
> > > > 
> > > > The perf time stamps approach the maximum precision of double (12 vs
> > > > 15 digits). Likely the division loses too many digits, which may cause
> > > > the bogus results. I've ran into similar problems before.
> > > 
> > > I don't think so, just look at the raw values:
> > > 
> > > $ perf script -F time,period --ns
> > > 71789.438122347:          1
> > > 71789.438127160:          1
> > > 71789.438129599:          7
> > > 71789.438131844:         94
> > > 71789.438134282:       1391
> > > 71789.438139871:      19106
> > > 71789.438156426:     123336
> > > ...
> > > 
> > > $ qalc '123336/(71789.438156426s - 71789.438139871s) to Hz'
> > > 123336 / ((71789.438 * second) - (71789.438 * second)) = approx.
> > > 7.4500755E9 Hz
> > > 
> > > > One way around is is to normalize the time stamps first that they
> > > > start with 0, but this only works for shorter traces.
> > > > Or use some bignum float library
> > > 
> > > I take the time delta between two samples, so a normalization of the
> > > individual times to 0 would not affect my calculations - the delta stays
> > > the same after all.
> > > 
> > > Also, using bignum in my calculations wouldn't change anything either. If
> > > perf tells me that 123336 cycles have been executed in 16.555 us, then it
> > > will always be larger than any expected value. At 3.2GHz it should be
> > > maximally 52976 cycles in such a short timeframe...
> > > 
> > > > Also at the beginning of frequency the periods are very small, and
> > > > the default us resolution will give big jumps for such a calculation.
> > > 
> > > OK, but who/what measures the large cycle values then? Is this a PMU
> > > limitation? Or is this an issue with the interaction with the kernel, when
> > > the algorithm tries to find a good frequency at the beginning?
> > > 
> > > > It's better to use the script --ns option then, but that makes the
> > > > double precision problem event worse.
> > > 
> > > See above, using `--ns` doesn't change anything. And qalc e.g. already
> > > uses
> > > bignum internally.
> > > 
> > > > In generally you get better results by avoiding frequency mode,
> > > > but always specify a fixed period.
> > > 
> > > This indeed removes the spikes at the beginning:
> > > 
> > > perf record --switch-events --call-graph dwarf -P -c 500000
> > > 
> > > The value is chosen to give a similar sample count to frequency mode.
> > 
> > Hey all,
> > 
> > I want to revive the above discussion as I'm still completely puzzled by the
> > observation. The tl;dr; for those who have not followed the previous
> > discussion:
> > 
> > perf record in frequency mode (i.e. "record ~1000 samples per second")
> > sometimes reports excessively large cycle counts.
> > 
> > In the previous mails I have outlined how to visualize this issue
> > graphically with gnuplot, by drawing the first derivative of the cycles
> > over time which gives nicely comparable numbers to ones CPU clock speed.
> > 
> > Sometimes, this value goe up to 10Ghz and beyond. Sometimes the values are
> > so broken (i.e. so high), that they completely break the analysis with perf
> > report or similar, as they completely skew the total event count and
> > thereby drastically influence the fractional cost reported by perf. E.g.
> > just now I ran `perf record` on another application and got this result:
> > 
> > ~~~~~
> > $ perf script | grep page_remove_rmap -C 10
> >  QXcbEventReader 23866 605019.879412:     128193 cycles:ppp: 
> > ffffffffbb23fbf5 __fget_light (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> >   lab_mandelbrot 23865 605019.879469:          1 cycles:ppp: 
> > ffffffffbb064504 native_write_msr
> > (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> >  QXcbEventReader 23866 605019.879471:    1810360 cycles:ppp: 
> > ffffffffbb1cb4ec find_vma (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> >   lab_mandelbrot 23865 605019.879472:          1 cycles:ppp: 
> > ffffffffbb064504 native_write_msr
> > (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> >   lab_mandelbrot 23865 605019.879474:         10 cycles:ppp: 
> > ffffffffbb064504 native_write_msr
> > (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> >   lab_mandelbrot 23865 605019.879475:        216 cycles:ppp: 
> > ffffffffbb064504 native_write_msr
> > (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> >   lab_mandelbrot 23865 605019.879486:       5106 cycles:ppp: 
> > ffffffffbb064504 native_write_msr
> > (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> >   lab_mandelbrot 23865 605019.879489:      19381 cycles:ppp:     
> > 7f85beae64ce QWaitCondition::wait (/usr/lib/libQt5Core.so.5.9.1)
> >   lab_mandelbrot 23865 605019.879495:     251346 cycles:ppp:     
> > 7f85bf2567c1 QScreen::~QScreen (/usr/lib/libQt5Gui.so.5.9.1)
> >   lab_mandelbrot 23865 605019.880722:    3210571 cycles:ppp:     
> > 7f85bd96f850 __cxa_finalize (/usr/lib/libc-2.25.so)
> >   lab_mandelbrot 23865 605019.881592: 21110358010774 cycles:ppp:
> > ffffffffbb1d4218 page_remove_rmap (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
> > ~~~~~
> > 
> > Note the last sample's cycle cost of 21110358010774. This is so large, that
> > it completely dominates the total event count, which lies at
> > 21126914067278. The runtime for this perf record was about 4s, it was on a
> > single threaded application with 4.12.8-2-ARCH on a Intel(R) Core(TM)
> > i7-4770 CPU @ 3.40GHz. So that hardware should at most be capable of
> > running up to ~1.36E10 cycles over 4s. perf record, thanks to the last
> > sample, measured ~2.11E13 cycles - clearly off the charts.
> > 
> > I have never seen this issue outside of perf's frequency mode. But then
> > again, that mode is the default and quite useful. Can anyone explain what
> > I'm seeing here?
> > 
> > Is it a bug in the kernel?
> > Is it a bug/limitation in the PMU?
> 
> I think this also sometimes manifests itself in lack of samples. I.e. for the 
> same workload above I now only get a couple dozen samples over a timeframe of 
> 4s in total:
> 
> ~~~~~
> $ time perf record  .../lab_mandelbrot -b 10
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.041 MB perf.data (131 samples) ]
> 
> real    0m5.042s
> user    0m4.964s
> sys     0m0.054s
> ~~~~~
> 
> Dmesg is silent here, so I don't think it's due to throttling:
> 
> ~~~~~
> /proc/sys/kernel/perf_cpu_time_max_percent
> 25
> /proc/sys/kernel/perf_event_max_contexts_per_stack
> 8
> /proc/sys/kernel/perf_event_max_sample_rate
> 50400
> /proc/sys/kernel/perf_event_max_stack
> 127
> /proc/sys/kernel/perf_event_mlock_kb
> 516
> /proc/sys/kernel/perf_event_paranoid
> -1
> ~~~~~
> 
> Rather, it's again the broken cycle counts which probably confuse the 
> frequency algorithm in the kernel:
> 
> ~~~~~
> $ perf script
>             perf  5678 611709.152451:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>             perf  5678 611709.152455:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>             perf  5678 611709.152456:          8 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>             perf  5678 611709.152457:        203 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>             perf  5678 611709.152459:       5421 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.152461:     144518 cycles:ppp:  ffffffffbb173fe0 
> __perf_event__output_id_sample (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.152502:    1902383 cycles:ppp:  ffffffffbb208e00 
> unlock_page_memcg (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.153037: 18471563529604 cycles:ppp:      
> 7f20085ecc58 _dl_map_object_from_fd (/usr/lib/ld-2.25.so)
>   lab_mandelbrot  5679 611709.163192:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5679 611709.163195:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5679 611709.163197:          7 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5679 611709.163198:        158 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5679 611709.163200:       3376 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5679 611709.163202:      79849 cycles:ppp:      7f2005e50b10 
> __ctype_init (/usr/lib/libc-2.25.so)
>   lab_mandelbrot  5679 611709.163233:    1142185 cycles:ppp:  ffffffffbb340a87 
> clear_page_erms (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164503:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164505:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164507:         14 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164508:        264 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164510:       5234 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164513:     102113 cycles:ppp:  ffffffffbb10928a 
> sys_futex (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164540:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164543:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164545:         12 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164547:        217 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164548:       4154 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.164551:      82262 cycles:ppp:      7f2006a880a2 
> pthread_cond_wait@@GLIBC_2.3.2 (/usr/lib/libpthread-2.25.so)
> 
> .. this pattern repeats and already looks quite bogus I think ...
> .. eventually we hit completely broken values: ...
> 
>   lab_mandelbrot  5678 611709.167097:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.167099:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.167100:         17 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.167102:        435 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.167103:      11560 cycles:ppp:  ffffffffbb031e7b 
> nmi_handle (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.167107:     306222 cycles:ppp:      7f2006a848ef 
> pthread_mutex_lock (/usr/lib/libpthread-2.25.so)
>   lab_mandelbrot  5678 611709.167277:    2558928 cycles:ppp:  ffffffffbb1bd9f5 
> vmacache_update (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.168034:    2710380 cycles:ppp:      7f2005e9e72a 
> _int_free (/usr/lib/libc-2.25.so)
>   lab_mandelbrot  5678 611709.168772:    2483393 cycles:ppp:  ffffffffbb220550 
> get_empty_filp (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.169450: 23749385103571 cycles:ppp:      
> 7f20086019d4 strcmp (/usr/lib/ld-2.25.so)
>   lab_mandelbrot  5678 611709.190936:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.190939:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.190941:          9 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.190942:        194 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.190943:       4357 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.190946:     100515 cycles:ppp:  ffffffffbb1086c3 
> do_futex (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>   lab_mandelbrot  5678 611709.190975:    1437218 cycles:ppp:      7f2005ea0ef4 
> _int_realloc (/usr/lib/libc-2.25.so)
>  QDBusConnection  5680 611709.191013:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>  QDBusConnection  5680 611709.191015:          1 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>  QDBusConnection  5680 611709.191026:         13 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>  QDBusConnection  5680 611709.191027:         50 cycles:ppp:  ffffffffbb064504 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>  QDBusConnection  5680 611709.191028:       1395 cycles:ppp:  ffffffffbb064506 
> native_write_msr (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>  QDBusConnection  5680 611709.191029:      38641 cycles:ppp:  ffffffffbb65bce7 
> schedule_hrtimeout_range_clock (/lib/modules/4.12.8-2-ARCH/build/vmlinux)
>  QDBusConnection  5680 611709.191041:     934282 cycles:ppp:      7f1ffcde715f 
> _dbus_first_type_in_signature (/usr/lib/libdbus-1.so.3.14.13)
>   lab_mandelbrot  5678 611709.191535:    2815425 cycles:ppp:      7f1ff46dc8ef 
> qMax<double> (/ssd2/milian/projects/compiled/kf5/lib/libKF5GuiAddons.so.
> 5.37.0)
>   lab_mandelbrot  5678 611709.192280:    2620652 cycles:ppp:      7f200640230d 
> [unknown] (/usr/lib/libm-2.25.so)
>   lab_mandelbrot  5678 611709.192992: 23749385195574 cycles:ppp:      
> 7f1ff644e442 QMap<KEntryKey, KEntry>::const_iterator::const_iterator (/ssd2/
> milian/projects/compiled/kf5/lib/libKF5ConfigCore.so.5.37.0)
>   lab_mandelbrot  5678 611709.536988:   27340128 cycles:ppp:      565094e15c28 
> drawMandelbrot (/ssd/milian/projects/kdab/training-material/addon/profiling/
> build/lab_mandelbrot/src/lab_mandelbrot)
>   lab_mandelbrot  5678 611709.544397: 2308960570885 cycles:ppp:      
> 7f20064029fc __hypot_finite (/usr/lib/libm-2.25.so)
>   lab_mandelbrot  5678 611709.858996:   19754129 cycles:ppp:      7f2006402a37 
> __hypot_finite (/usr/lib/libm-2.25.so)
>   lab_mandelbrot  5678 611709.864538: 2308954751998 cycles:ppp:      
> 7f2006402a77 __hypot_finite (/usr/lib/libm-2.25.so)
>   lab_mandelbrot  5678 611710.183972:   14591525 cycles:ppp:      565094e14ce0 
> cabs@plt (/ssd/milian/projects/kdab/training-material/addon/profiling/build/
> lab_mandelbrot/src/lab_mandelbrot)
>   lab_mandelbrot  5678 611710.188004: 2638800790961 cycles:ppp:      
> 7f20078dacc0 QColor::toRgb (/usr/lib/libQt5Gui.so.5.9.1)
>   lab_mandelbrot  5678 611710.495016:   25617507 cycles:ppp:      7f20061cdc6d 
> __muldc3 (/usr/lib/libgcc_s.so.1)
>   lab_mandelbrot  5678 611710.501962: 2308959251991 cycles:ppp:      
> 7f2006402a63 __hypot_finite (/usr/lib/libm-2.25.so)
>   lab_mandelbrot  5678 611710.815734:   19739740 cycles:ppp:      7f20061cdc5c 
> __muldc3 (/usr/lib/libgcc_s.so.1)
>   lab_mandelbrot  5678 611710.821164: 2308954745231 cycles:ppp:      
> 7f2006402b33 __hypot_finite (/usr/lib/libm-2.25.so)
> ~~~~~
> 
> So, is my PMU messed up?
> -- 
> Milian Wolff | milian.wolff@xxxxxxxx | Senior Software Engineer
> KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
> Tel: +49-30-521325470
> KDAB - The Qt Experts