From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752394AbeCXWD2 (ORCPT ); Sat, 24 Mar 2018 18:03:28 -0400 Received: from orthanc.universe-factory.net ([104.238.176.138]:34814 "EHLO orthanc.universe-factory.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752167AbeCXWD1 (ORCPT ); Sat, 24 Mar 2018 18:03:27 -0400 Subject: Re: [PATCH] ftrace: fix stddev calculation in function profiler (again) To: rostedt@goodmis.org, mingo@redhat.com Cc: linux-kernel@vger.kernel.org References: From: Matthias Schiffer Message-ID: Date: Sat, 24 Mar 2018 23:03:24 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="QIcSdTNdQbVUmkMYhAU2V7CSMryMLcVoU" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --QIcSdTNdQbVUmkMYhAU2V7CSMryMLcVoU Content-Type: multipart/mixed; boundary="gNe82BcB85CBDAM5Sw5K5mHqT6UTnAY85"; protected-headers="v1" From: Matthias Schiffer To: rostedt@goodmis.org, mingo@redhat.com Cc: linux-kernel@vger.kernel.org Message-ID: Subject: Re: [PATCH] ftrace: fix stddev calculation in function profiler (again) References: In-Reply-To: --gNe82BcB85CBDAM5Sw5K5mHqT6UTnAY85 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 03/24/2018 05:26 PM, Matthias Schiffer wrote: > It is well-known that it is not possible to accurately calculate varian= ces > just by accumulating squared samples; in fact, such an approach can eve= n > result in negative numbers. An earlier attempt to fix the calculation > referred to Welford's method, but did not implement it correctly, leadi= ng > to meeaningless output like the following: >=20 > nf_conntrack_proto_fini 50 373.523 us 7.470 us 323431595= 1 us >=20 > Welford's method uses one do_div() in the tracing path; this cannot be > avoided. The average time is added to struct ftrace_profile, so only a > single division is required. I also considered the following alternativ= es: >=20 > 1) Only keeping the avg field and removing the aggregated time would > greatly lead to severe rounding errors in calculating the total time ba= sed > on counter and avg. >=20 > 2) Calculating both the old and the new average in profile_graph_return= () > instead of storing it in struct ftrace_profile would require a second > division. >=20 > 3) I managed to transform Welford's equations in a way that uses the to= tal > time instead of the average and requires only a single division. > Unfortunately, the divisor is counter^3 in this case, easily overflowin= g > even 64bit integers. >=20 > Ruling out the above alternatives, I chose the present approach to fix = the > issue. >=20 > Fixes: e330b3bcd831 ("tracing: Show sample std dev in function profilin= g") > Fixes: 52d85d763086 ("ftrace: Fix stddev calculation in function profil= er") > Signed-off-by: Matthias Schiffer Hmm, in further testing, I'm seeing profile_graph_return() being called without rec->counter getting incremented a lot, completely throwing off t= he stddev calculation (and also slightly displacing the average). Is this expected? Should we possibly move the counter increment to profile_graph_return() i= n the CONFIG_FUNCTION_GRAPH_TRACER case, or introduce a second counter for returns? Matthias > --- > kernel/trace/ftrace.c | 33 +++++++++++++++++++-------------- > 1 file changed, 19 insertions(+), 14 deletions(-) >=20 > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > index eac9ce2c57a2..16dce67b855a 100644 > --- a/kernel/trace/ftrace.c > +++ b/kernel/trace/ftrace.c > @@ -460,7 +460,8 @@ struct ftrace_profile { > unsigned long counter; > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > unsigned long long time; > - unsigned long long time_squared; > + unsigned long long avg; > + unsigned long long stddev; > #endif > }; > =20 > @@ -580,7 +581,6 @@ static int function_stat_show(struct seq_file *m, v= oid *v) > int ret =3D 0; > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > static struct trace_seq s; > - unsigned long long avg; > unsigned long long stddev; > #endif > mutex_lock(&ftrace_profile_lock); > @@ -592,9 +592,7 @@ static int function_stat_show(struct seq_file *m, v= oid *v) > } > =20 > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > - avg =3D rec->time; > - do_div(avg, rec->counter); > - if (tracing_thresh && (avg < tracing_thresh)) > + if (tracing_thresh && (rec->avg < tracing_thresh)) > goto out; > #endif > =20 > @@ -608,24 +606,19 @@ static int function_stat_show(struct seq_file *m,= void *v) > if (rec->counter <=3D 1) > stddev =3D 0; > else { > - /* > - * Apply Welford's method: > - * s^2 =3D 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2) > - */ > - stddev =3D rec->counter * rec->time_squared - > - rec->time * rec->time; > + stddev =3D rec->stddev; > =20 > /* > * Divide only 1000 for ns^2 -> us^2 conversion. > * trace_print_graph_duration will divide 1000 again. > */ > - do_div(stddev, rec->counter * (rec->counter - 1) * 1000); > + do_div(stddev, 1000 * (rec->counter - 1)); > } > =20 > trace_seq_init(&s); > trace_print_graph_duration(rec->time, &s); > trace_seq_puts(&s, " "); > - trace_print_graph_duration(avg, &s); > + trace_print_graph_duration(rec->avg, &s); > trace_seq_puts(&s, " "); > trace_print_graph_duration(stddev, &s); > trace_print_seq(m, &s); > @@ -905,8 +898,20 @@ static void profile_graph_return(struct ftrace_gra= ph_ret *trace) > =20 > rec =3D ftrace_find_profiled_func(stat, trace->func); > if (rec) { > + unsigned long long avg, delta1, delta2; > + > rec->time +=3D calltime; > - rec->time_squared +=3D calltime * calltime; > + > + /* Apply Welford's method */ > + delta1 =3D calltime - rec->avg; > + > + avg =3D rec->time; > + do_div(avg, rec->counter); > + rec->avg =3D avg; > + > + delta2 =3D calltime - rec->avg; > + > + rec->stddev +=3D delta1 * delta2; > } > =20 > out: >=20 --gNe82BcB85CBDAM5Sw5K5mHqT6UTnAY85-- --QIcSdTNdQbVUmkMYhAU2V7CSMryMLcVoU Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEZmTnvaa2aYgexS51Fu8/ZMsgHZwFAlq2yywACgkQFu8/ZMsg HZzUlQ//V+2NjGmqt0VsmuY8RnPrbRQ5KOb9/tQVL04Hs7Fz81KtSjPjEqzg7gJj S8bTG/lcIGnIi3SDNITW1vDuA3SlSMO+sZ5cQStQ0le+uFJ/QkLv/AXMyCtDZHeX mSsra6yMELuyHzA+j8KwseWuAKDih6ufM0d2B4deRHow4qzH7wgMCU/fNDHew5LH JmezDPzSPJqRzFnrjDpUpfS5qNDTdx04PqyB22lBIwe6gvpzb4d/Q18YdhXVog6j sOQEdHKKUhwmVcHjpMrTFcMuO1uDvQvk0NIRyTRE6bNaP6hLukasvjPPDexUiBKF EUJ/faUkqZmutdUV/0o+/TzAkw8RuDcrGtlELS4L4b2u82XkGSu4WvfJPN9oAgCU fcuKpJBO2apUkHDb3BPg6dtrkID28LhE6nHmQTVhcotrepUlOgJ3MeoPd6L9hb9b 7/qXU8JtMohapVPeodnHU6RLOoxaDVXF7+Po9GlQwsKDlJFsJvcDUt1s5a+Fkw/B NYH2N4gko9HjyYPMsictFmz23/HQARQ3Gnw9ehX1wtBg3shyBX+Wz6foZVCZ4oeX zd3UDi124IOu3ES+r0LBvVMKcuyRwCJ/iwQyCP/BBGVkSfjEAy6xYuOYnFW2PvI/ 2lTwyPzTaExe5ETPQ5ZSxAUktDSh/bK+Ftxr8Yfe7bPmhKQ1snQ= =70Z9 -----END PGP SIGNATURE----- --QIcSdTNdQbVUmkMYhAU2V7CSMryMLcVoU--