Re: [PATCH v3 2/2] service: fix potential stats race-condition on MT services

[David Marchand <david.marchand@redhat.com>]

On Mon, Jul 11, 2022 at 3:18 PM Harry van Haaren
<harry.van.haaren@intel.com> 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.
> Regular reads and increments are used, and only the store is
> specified as atomic, reducing perf impact on e.g. x86 arch.
>
> 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.

Fixes: 21698354c832 ("service: introduce service cores concept")

I did not mark for backport since the commitlog indicates a performance impact.
You can still ask for backport by pinging LTS maintainers.

>
> Reported-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> Suggested-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Suggested-by: Morten Brørup <mb@smartsharesystems.com>
> Suggested-by: Bruce Richardson <bruce.richardson@intel.com>
> Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>


Series applied, thanks.

-- 
David Marchand