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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: Feifei Wang <feifei.wang2@arm.com>, Honnappa Nagarahalli
<honnappa.nagarahalli@arm.com>
CC: "dev@dpdk.org" <dev@dpdk.org>, "nd@arm.com" <nd@arm.com>,
"Honnappa.Nagarahalli@arm.com" <Honnappa.Nagarahalli@arm.com>,
"ruifeng.wang@arm.com" <ruifeng.wang@arm.com>
Thread-Topic: [PATCH v1 1/2] app/test: remove unnecessary barriers for ring
stress test
Thread-Index: AQHW2CwQ96/sxptInkmsLdK5Uf3RAKoDBLrQ
Date: Tue, 22 Dec 2020 12:42:26 +0000
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References: <20201222063054.44429-1-feifei.wang2@arm.com>
<20201222063054.44429-2-feifei.wang2@arm.com>
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Subject: Re: [dpdk-dev] [PATCH v1 1/2] app/test: remove unnecessary barriers
for ring stress test
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Hi Feifei,
>=20
> The variable "wrk_cmd" is a signal to control threads from running and
> stopping. When worker lcores load "wrk_cmd =3D=3D WRK_CMD_RUN", they star=
t
> running and when worker lcores load "wrk_cmd =3D=3D WRK_CMD_STOP", they
> stop.
>=20
> For the wmb in test_mt1, no storing operations must keep the order
> after storing "wrk_cmd". Thus the wmb is unnecessary.
I think there is a bug in my original code, we should do smp_wmb() *before*=
=20
setting wrk_cmd, not after:
/* launch on all workers */
RTE_LCORE_FOREACH_WORKER(lc) {
arg[lc].rng =3D r;
arg[lc].stats =3D init_stat;
rte_eal_remote_launch(test, &arg[lc], lc);
}
/* signal worker to start test */
+ rte_smp_wmb();
wrk_cmd =3D WRK_CMD_RUN;
- rte_smp_wmb();
usleep(run_time * US_PER_S);
I still think we'd better have some synchronisation here.
Otherwise what would prevent compiler and/or cpu to update wrk_cmd out of o=
rder
(before _init_ phase is completed)?
We probably can safely assume no reordering from the compiler here,
as we have function calls straight before and after 'wrk_cmd =3D WRK_CMD_RU=
N;'
But for consistency and easier maintenance, I still think it is better
to have something here, after all it is not performance critical pass.=20
> For the rmb in test_worker, the parameters have been prepared when
> worker lcores call "test_worker". It is unnessary to wait wrk_cmd to be
> loaded, then the parameters can be loaded, So the rmb can be removed.
It is not only about parameters loading, it is to prevent worker core to s=
tart too early.
As I understand, your goal is to get rid of rte_smp_*() calls.
Might be better to replace such places here with _atomic_ semantics.
Then, as I can see, we also can get rid of 'volatile' fo wrk_cmd.
=20
> In the meanwhile, fix a typo. The note above storing "stop" into
> "wrk_cmd" should be "stop test" rather than "start test".
>=20
> Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
> Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
> app/test/test_ring_stress_impl.h | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>=20
> diff --git a/app/test/test_ring_stress_impl.h b/app/test/test_ring_stress=
_impl.h
> index f9ca63b90..384555ef9 100644
> --- a/app/test/test_ring_stress_impl.h
> +++ b/app/test/test_ring_stress_impl.h
> @@ -198,7 +198,6 @@ test_worker(void *arg, const char *fname, int32_t prc=
s)
> fill_ring_elm(&loc_elm, lc);
>=20
> while (wrk_cmd !=3D WRK_CMD_RUN) {
> - rte_smp_rmb();
> rte_pause();
> }
>=20
> @@ -357,13 +356,11 @@ test_mt1(int (*test)(void *))
>=20
> /* signal worker to start test */
> wrk_cmd =3D WRK_CMD_RUN;
> - rte_smp_wmb();
>=20
> usleep(run_time * US_PER_S);
>=20
> - /* signal worker to start test */
> + /* signal worker to stop test */
> wrk_cmd =3D WRK_CMD_STOP;
> - rte_smp_wmb();
>=20
> /* wait for workers and collect stats. */
> mc =3D rte_lcore_id();
> --
> 2.17.1