[dpdk-dev] [PATCH 2/3] eventdev: use c11 atomics for lcore timer armed flag

[Phil Yang]

Hi Erik,

Sorry, I was on vacation.
Thanks for your feedback. I will update it in the next version.

Thanks,
Phil

> -----Original Message-----
> From: Carrillo, Erik G <erik.g.carrillo at intel.com>
> Sent: Wednesday, June 24, 2020 5:02 AM
> To: Phil Yang <Phil.Yang at arm.com>; dev at dpdk.org
> Cc: drc at linux.vnet.ibm.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli at arm.com>; Ruifeng Wang
> <Ruifeng.Wang at arm.com>; Dharmik Thakkar <Dharmik.Thakkar at arm.com>;
> nd <nd at arm.com>
> Subject: RE: [PATCH 2/3] eventdev: use c11 atomics for lcore timer armed
> flag
> 
> Hi Phil,
> 
> Comment in-line:
> 
> > -----Original Message-----
> > From: Phil Yang <phil.yang at arm.com>
> > Sent: Friday, June 12, 2020 6:20 AM
> > To: dev at dpdk.org; Carrillo, Erik G <erik.g.carrillo at intel.com>
> > Cc: drc at linux.vnet.ibm.com; honnappa.nagarahalli at arm.com;
> > ruifeng.wang at arm.com; dharmik.thakkar at arm.com; nd at arm.com
> > Subject: [PATCH 2/3] eventdev: use c11 atomics for lcore timer armed flag
> >
> > The in_use flag is a per core variable which is not shared between lcores in
> > the normal case and the access of this variable should be ordered on the
> > same core. However, if non-EAL thread pick the highest lcore to insert
> timers
> > into, there is the possibility of conflicts on this flag between threads. Then
> > the atomic CAS operation is needed.
> >
> > Use the c11 atomic CAS instead of the generic rte_atomic operations to
> avoid
> > the unnecessary barrier on aarch64.
> >
> > Signed-off-by: Phil Yang <phil.yang at arm.com>
> > Reviewed-by: Dharmik Thakkar <dharmik.thakkar at arm.com>
> > Reviewed-by: Ruifeng Wang <ruifeng.wang at arm.com>
> > ---
> >  lib/librte_eventdev/rte_event_timer_adapter.c | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/lib/librte_eventdev/rte_event_timer_adapter.c
> > b/lib/librte_eventdev/rte_event_timer_adapter.c
> > index 6a0e283..6947efb 100644
> > --- a/lib/librte_eventdev/rte_event_timer_adapter.c
> > +++ b/lib/librte_eventdev/rte_event_timer_adapter.c
> > @@ -554,7 +554,7 @@ struct swtim {
> >  	uint32_t timer_data_id;
> >  	/* Track which cores have actually armed a timer */
> >  	struct {
> > -		rte_atomic16_t v;
> > +		int16_t v;
> >  	} __rte_cache_aligned in_use[RTE_MAX_LCORE];
> >  	/* Track which cores' timer lists should be polled */
> >  	unsigned int poll_lcores[RTE_MAX_LCORE]; @@ -606,7 +606,8 @@
> > swtim_callback(struct rte_timer *tim)
> >  				      "with immediate expiry value");
> >  		}
> >
> > -		if (unlikely(rte_atomic16_test_and_set(&sw-
> > >in_use[lcore].v))) {
> > +		if (unlikely(sw->in_use[lcore].v == 0)) {
> > +			sw->in_use[lcore].v = 1;
> >  			n_lcores = __atomic_fetch_add(&sw->n_poll_lcores,
> > 1,
> >  						__ATOMIC_RELAXED);
> >  			__atomic_store_n(&sw->poll_lcores[n_lcores],
> > lcore, @@ -834,7 +835,7 @@ swtim_init(struct rte_event_timer_adapter
> > *adapter)
> >
> >  	/* Initialize the variables that track in-use timer lists */
> >  	for (i = 0; i < RTE_MAX_LCORE; i++)
> > -		rte_atomic16_init(&sw->in_use[i].v);
> > +		sw->in_use[i].v = 0;
> >
> >  	/* Initialize the timer subsystem and allocate timer data instance */
> >  	ret = rte_timer_subsystem_init();
> > @@ -1017,6 +1018,8 @@ __swtim_arm_burst(const struct
> > rte_event_timer_adapter *adapter,
> >  	struct rte_timer *tim, *tims[nb_evtims];
> >  	uint64_t cycles;
> >  	int n_lcores;
> > +	/* Timer is not armed state */
> 
> A more accurate comment would be something like "Timer list for this lcore is
> not in use".
> 
> With that change, it looks good to me:
> Acked-by: Erik Gabriel Carrillo <erik.g.carrillo at intel.com>
> 
> > +	int16_t exp_state = 0;
> >
> >  #ifdef RTE_LIBRTE_EVENTDEV_DEBUG
> >  	/* Check that the service is running. */ @@ -1035,8 +1038,12 @@
> > __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
> >  	/* If this is the first time we're arming an event timer on this lcore,
> >  	 * mark this lcore as "in use"; this will cause the service
> >  	 * function to process the timer list that corresponds to this lcore.
> > +	 * The atomic CAS operation can prevent the race condition on
> > in_use
> > +	 * flag between multiple non-EAL threads.
> >  	 */
> > -	if (unlikely(rte_atomic16_test_and_set(&sw->in_use[lcore_id].v))) {
> > +	if (unlikely(__atomic_compare_exchange_n(&sw-
> > >in_use[lcore_id].v,
> > +			&exp_state, 1, 0,
> > +			__ATOMIC_RELAXED, __ATOMIC_RELAXED))) {
> >  		EVTIM_LOG_DBG("Adding lcore id = %u to list of lcores to
> > poll",
> >  			      lcore_id);
> >  		n_lcores = __atomic_fetch_add(&sw->n_poll_lcores, 1,
> > --
> > 2.7.4