[PATCH 2/2] service: fix potential stats race-condition on MT services

[Mattias Rönnblom]

On 2022-07-08 14:56, Harry van Haaren wrote:
> 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 at ericsson.com>
> Suggested-by: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
> Suggested-by: Morten Brørup <mb at smartsharesystems.com>
> Signed-off-by: Harry van Haaren <harry.van.haaren at 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++;
> +		}
>   	} else
>   		s->spec.callback(userdata);
>   }

Reviewed-by: Mattias Rönnblom <mattias.ronnblom at ericsson.com>

Thanks for the fix Harry.