Re: [dpdk-dev] [PATCH v1 1/2] app/test: remove unnecessary barriers for ring stress test

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Feifei,

> 
> The variable "wrk_cmd" is a signal to control threads from running and
> stopping. When worker lcores load "wrk_cmd == WRK_CMD_RUN", they start
> running and when worker lcores load "wrk_cmd == WRK_CMD_STOP", they
> stop.
> 
> 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*  
setting wrk_cmd, not after:

        /* launch on all workers */
        RTE_LCORE_FOREACH_WORKER(lc) {
                arg[lc].rng = r;
                arg[lc].stats = init_stat;
                rte_eal_remote_launch(test, &arg[lc], lc);
        }

        /* signal worker to start test */
+      rte_smp_wmb();
        wrk_cmd = 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 order
(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 = WRK_CMD_RUN;'
But for consistency and easier maintenance, I still think it is better
to have something here, after all it is not performance critical pass. 

> 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 start 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.
 
> In the meanwhile, fix a typo. The note above storing "stop" into
> "wrk_cmd" should be "stop test" rather than "start test".
> 
> 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(-)
> 
> 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 prcs)
>  	fill_ring_elm(&loc_elm, lc);
> 
>  	while (wrk_cmd != WRK_CMD_RUN) {
> -		rte_smp_rmb();
>  		rte_pause();
>  	}
> 
> @@ -357,13 +356,11 @@ test_mt1(int (*test)(void *))
> 
>  	/* signal worker to start test */
>  	wrk_cmd = WRK_CMD_RUN;
> -	rte_smp_wmb();
> 
>  	usleep(run_time * US_PER_S);
> 
> -	/* signal worker to start test */
> +	/* signal worker to stop test */
>  	wrk_cmd = WRK_CMD_STOP;
> -	rte_smp_wmb();
> 
>  	/* wait for workers and collect stats. */
>  	mc = rte_lcore_id();
> --
> 2.17.1