From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: "Morten Brørup" <mb@smartsharesystems.com>,
"Harry van Haaren" <harry.van.haaren@intel.com>,
"dev@dpdk.org" <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 16:45:57 +0000 [thread overview]
Message-ID: <DBAPR08MB581482CA84AA4D5C033B9F3B98829@DBAPR08MB5814.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35D871B8@smartserver.smartshare.dk>
<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. :-)
I am thinking of the case where the 64b write is split into two 32b (or more) write operations either by the compiler or the micro-architecture. If this were to happen, it causes race conditions with the reader.
As far as I understand, the compiler does not provide any guarantees on generating non-tearing stores unless an atomic builtin/function is used. If we have to ensure the micro-architecture does not generate split writes, we need to be careful that future code additions do not change the alignment of the stats.
>
> >
> > > + }
> > > } else
> > > s->spec.callback(userdata);
> > > }
> > > --
> > > 2.32.0
next prev parent reply other threads:[~2022-07-08 16:46 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
2022-07-08 16:45 ` Honnappa Nagarahalli [this message]
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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