From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Honnappa Nagarahalli" <Honnappa.Nagarahalli@arm.com>,
"Harry van Haaren" <harry.van.haaren@intel.com>, <dev@dpdk.org>
Cc: "Mattias Rönnblom" <mattias.ronnblom@ericsson.com>,
nd <nd@arm.com>, nd <nd@arm.com>
Subject: RE: [PATCH 2/2] service: fix potential stats race-condition on MT services
Date: Fri, 8 Jul 2022 18:29:42 +0200 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D871B8@smartserver.smartshare.dk> (raw)
In-Reply-To: <DBAPR08MB58145179979AD3BDA401FE4798829@DBAPR08MB5814.eurprd08.prod.outlook.com>
> From: Honnappa Nagarahalli [mailto:Honnappa.Nagarahalli@arm.com]
> Sent: Friday, 8 July 2022 17.16
>
> <snip>
> >
> > This commit fixes a potential racey-add that could occur if multiple
> service-
> > lcores were executing the same MT-safe service at the same time, with
> > service statistics collection enabled.
> >
> > Because multiple threads can run and execute the service, the stats
> values
> > can have multiple writer threads, resulting in the requirement of
> using
> > atomic addition for correctness.
> >
> > Note that when a MT unsafe service is executed, a spinlock is held,
> so the
> > stats increments are protected. This fact is used to avoid executing
> atomic
> > add instructions when not required.
> >
> > This patch causes a 1.25x increase in cycle-cost for polling a MT
> safe service
> > when statistics are enabled. No change was seen for MT unsafe
> services, or
> > when statistics are disabled.
> >
> > Reported-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> > Suggested-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> > Suggested-by: Morten Brørup <mb@smartsharesystems.com>
> > Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
> >
> > ---
> > ---
> > lib/eal/common/rte_service.c | 10 ++++++++--
> > 1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/eal/common/rte_service.c
> b/lib/eal/common/rte_service.c
> > index ef31b1f63c..f045e74ef3 100644
> > --- a/lib/eal/common/rte_service.c
> > +++ b/lib/eal/common/rte_service.c
> > @@ -363,9 +363,15 @@ service_runner_do_callback(struct
> > rte_service_spec_impl *s,
> > uint64_t start = rte_rdtsc();
> > s->spec.callback(userdata);
> > uint64_t end = rte_rdtsc();
> > - s->cycles_spent += end - start;
> > + uint64_t cycles = end - start;
> > cs->calls_per_service[service_idx]++;
> > - s->calls++;
> > + if (service_mt_safe(s)) {
> > + __atomic_fetch_add(&s->cycles_spent, cycles,
> > __ATOMIC_RELAXED);
> > + __atomic_fetch_add(&s->calls, 1,
> > __ATOMIC_RELAXED);
> > + } else {
> > + s->cycles_spent += cycles;
> > + s->calls++;
> This is still a problem from a reader perspective. It is possible that
> the writes could be split while a reader is reading the stats. These
> need to be atomic adds.
I don't understand what you suggest can go wrong here, Honnappa. If you talking about 64 bit counters on 32 bit architectures, then I understand the problem (and have many years of direct experience with it myself). Otherwise, I hope you can elaborate or direct me to educational material about the issue, considering this a learning opportunity. :-)
>
> > + }
> > } else
> > s->spec.callback(userdata);
> > }
> > --
> > 2.32.0
next prev parent reply other threads:[~2022-07-08 16:29 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-08 12:56 [PATCH 1/2] test/service: add perf measurements for with stats mode Harry van Haaren
2022-07-08 12:56 ` [PATCH 2/2] service: fix potential stats race-condition on MT services Harry van Haaren
2022-07-08 13:23 ` Morten Brørup
2022-07-08 13:44 ` Van Haaren, Harry
2022-07-08 14:14 ` Morten Brørup
2022-07-08 13:48 ` Mattias Rönnblom
2022-07-08 15:16 ` Honnappa Nagarahalli
2022-07-08 15:31 ` Van Haaren, Harry
2022-07-08 16:21 ` Bruce Richardson
2022-07-08 16:33 ` Honnappa Nagarahalli
2022-07-08 20:02 ` Mattias Rönnblom
2022-07-08 16:29 ` Morten Brørup [this message]
2022-07-08 16:45 ` Honnappa Nagarahalli
2022-07-08 17:22 ` Morten Brørup
2022-07-08 17:39 ` Honnappa Nagarahalli
2022-07-08 18:08 ` Morten Brørup
2022-09-06 16:13 ` [PATCH 1/6] service: reduce statistics overhead for parallel services Mattias Rönnblom
2022-09-06 16:13 ` [PATCH 2/6] service: introduce per-lcore cycles counter Mattias Rönnblom
2022-09-06 16:13 ` [PATCH 3/6] service: reduce average case service core overhead Mattias Rönnblom
2022-10-03 13:33 ` Van Haaren, Harry
2022-10-03 14:32 ` Mattias Rönnblom
2022-09-06 16:13 ` [PATCH 4/6] service: tweak cycle statistics semantics Mattias Rönnblom
2022-09-07 8:41 ` Morten Brørup
2022-10-03 13:45 ` Van Haaren, Harry
2022-09-06 16:13 ` [PATCH 5/6] event/sw: report idle when no work is performed Mattias Rönnblom
2022-09-06 16:13 ` [PATCH 6/6] service: provide links to functions in documentation Mattias Rönnblom
2022-10-03 8:06 ` [PATCH 1/6] service: reduce statistics overhead for parallel services David Marchand
2022-10-03 8:40 ` Mattias Rönnblom
2022-10-03 9:53 ` David Marchand
2022-10-03 11:37 ` Mattias Rönnblom
2022-10-03 13:03 ` Van Haaren, Harry
2022-10-03 13:33 ` Van Haaren, Harry
2022-10-03 14:37 ` Mattias Rönnblom
2022-10-05 9:16 ` [PATCH v2 0/6] Service cores performance and statistics improvements Mattias Rönnblom
2022-10-05 9:16 ` [PATCH v2 1/6] service: reduce statistics overhead for parallel services Mattias Rönnblom
2022-10-05 9:16 ` [PATCH v2 2/6] service: introduce per-lcore cycles counter Mattias Rönnblom
2022-10-05 9:16 ` [PATCH v2 3/6] service: reduce average case service core overhead Mattias Rönnblom
2022-10-05 9:16 ` [PATCH v2 4/6] service: tweak cycle statistics semantics Mattias Rönnblom
2022-10-05 9:16 ` [PATCH v2 5/6] event/sw: report idle when no work is performed Mattias Rönnblom
2022-10-05 9:16 ` [PATCH v2 6/6] service: provide links to functions in documentation Mattias Rönnblom
2022-10-05 9:49 ` [PATCH v2 0/6] Service cores performance and statistics improvements Morten Brørup
2022-10-05 10:14 ` Mattias Rönnblom
2022-10-05 13:39 ` David Marchand
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