Re: [dpdk-dev] [PATCH v3 11/12] service: optimize with c11 one-way barrier

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> > Subject: [PATCH v3 11/12] service: optimize with c11 one-way barrier
> >
> > The num_mapped_cores and execute_lock are synchronized with
> > rte_atomic_XX APIs which is a full barrier, DMB, on aarch64. This
> > patch optimized it with
> > c11 atomic one-way barrier.
> >
> > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> 
> Based on discussion on-list, it seems the consensus is to not use GCC builtins,
> but instead use C11 APIs "proper"? If my conclusion is correct, the v+1 of this
> patchset would require updates to that style API.
> 
> Inline comments for context below, -Harry
> 
> 
> > ---
> >  lib/librte_eal/common/rte_service.c | 50
> > ++++++++++++++++++++++++++----------
> > -
> >  1 file changed, 35 insertions(+), 15 deletions(-)
> >
> > diff --git a/lib/librte_eal/common/rte_service.c
> > b/lib/librte_eal/common/rte_service.c
> > index 0843c3c..c033224 100644
> > --- a/lib/librte_eal/common/rte_service.c
> > +++ b/lib/librte_eal/common/rte_service.c
> > @@ -42,7 +42,7 @@ struct rte_service_spec_impl {
> >  	 * running this service callback. When not set, a core may take the
> >  	 * lock and then run the service callback.
> >  	 */
> > -	rte_atomic32_t execute_lock;
> > +	uint32_t execute_lock;
> >
> >  	/* API set/get-able variables */
> >  	int8_t app_runstate;
> > @@ -54,7 +54,7 @@ struct rte_service_spec_impl {
> >  	 * It does not indicate the number of cores the service is running
> >  	 * on currently.
> >  	 */
> > -	rte_atomic32_t num_mapped_cores;
> > +	int32_t num_mapped_cores;
> 
> Any reason why "int32_t" or "uint32_t" is used over another?
> execute_lock is a uint32_t above, num_mapped_cores is an int32_t?
> 
> 
> >  	uint64_t calls;
> >  	uint64_t cycles_spent;
> >  } __rte_cache_aligned;
> > @@ -332,7 +332,8 @@ rte_service_runstate_get(uint32_t id)
> >  	rte_smp_rmb();
> >
> >  	int check_disabled = !(s->internal_flags & SERVICE_F_START_CHECK);
> > -	int lcore_mapped = (rte_atomic32_read(&s->num_mapped_cores) >
> 0);
> > +	int lcore_mapped = (__atomic_load_n(&s->num_mapped_cores,
> > +					    __ATOMIC_RELAXED) > 0);
> >
> >  	return (s->app_runstate == RUNSTATE_RUNNING) &&
> >  		(s->comp_runstate == RUNSTATE_RUNNING) && @@ -375,11
> +376,20 @@
> > service_run(uint32_t i, struct core_state *cs, uint64_t service_mask,
> >  	cs->service_active_on_lcore[i] = 1;
> >
> >  	if ((service_mt_safe(s) == 0) && (serialize_mt_unsafe == 1)) {
> > -		if (!rte_atomic32_cmpset((uint32_t *)&s->execute_lock, 0, 1))
> > +		uint32_t expected = 0;
> > +		/* ACQUIRE ordering here is to prevent the callback
> > +		 * function from hoisting up before the execute_lock
> > +		 * setting.
> > +		 */
> > +		if (!__atomic_compare_exchange_n(&s->execute_lock,
> &expected, 1,
> > +			    0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
> >  			return -EBUSY;
> 
> Let's try improve the magic "1" and "0" constants, I believe the "1" here is the
> desired "new value on success", and the 0 is "bool weak", where our 0/false
> constant implies a strongly ordered compare exchange?
> 
> "Weak is true for weak compare_exchange, which may fail spuriously, and
> false for the strong variation, which never fails spuriously.", from
> https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
> 
> const uint32_t on_success_value = 1;
> const bool weak = 0;
> __atomic_compare_exchange_n(&s->execute_lock, &expected,
> on_success_value, weak, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
> 
> 
> Although a bit more verbose, I feel this documents usage a lot better,
> particularly for those who aren't as familiar with the C11 function arguments
> order.
> 
> Admittedly with the API change to not use __builtins, perhaps this comment is
> moot.
Suggest changing the execute_lock to rte_spinlock_t and use rte_spinlock_trylock API.

> 
> 
> >
> >  		service_runner_do_callback(s, cs, i);
> > -		rte_atomic32_clear(&s->execute_lock);
> > +		/* RELEASE ordering here is used to pair with ACQUIRE
> > +		 * above to achieve lock semantic.
> > +		 */
> > +		__atomic_store_n(&s->execute_lock, 0, __ATOMIC_RELEASE);
> >  	} else
> >  		service_runner_do_callback(s, cs, i);
> >
> > @@ -415,11 +425,11 @@ rte_service_run_iter_on_app_lcore(uint32_t id,
> > uint32_t
> > serialize_mt_unsafe)
> >  	/* Increment num_mapped_cores to indicate that the service
> >  	 * is running on a core.
> >  	 */
> > -	rte_atomic32_inc(&s->num_mapped_cores);
> > +	__atomic_add_fetch(&s->num_mapped_cores, 1,
> __ATOMIC_ACQUIRE);
> >
> >  	int ret = service_run(id, cs, UINT64_MAX, s, serialize_mt_unsafe);
> >
> > -	rte_atomic32_dec(&s->num_mapped_cores);
> > +	__atomic_sub_fetch(&s->num_mapped_cores, 1,
> __ATOMIC_RELEASE);
> >
> >  	return ret;
> >  }
> > @@ -552,24 +562,32 @@ service_update(uint32_t sid, uint32_t lcore,
> >
> >  	uint64_t sid_mask = UINT64_C(1) << sid;
> >  	if (set) {
> > -		uint64_t lcore_mapped = lcore_states[lcore].service_mask &
> > -			sid_mask;
> > +		/* When multiple threads try to update the same lcore
> > +		 * service concurrently, e.g. set lcore map followed
> > +		 * by clear lcore map, the unsynchronized service_mask
> > +		 * values have issues on the num_mapped_cores value
> > +		 * consistency. So we use ACQUIRE ordering to pair with
> > +		 * the RELEASE ordering to synchronize the service_mask.
> > +		 */
> > +		uint64_t lcore_mapped = __atomic_load_n(
> > +					&lcore_states[lcore].service_mask,
> > +					__ATOMIC_ACQUIRE) & sid_mask;
> 
> Thanks for the comment - it helps me understand things a bit better.
> Some questions/theories to validate;
> 1) The service_mask ACQUIRE avoids other loads being hoisted above it,
> correct?
> 
> 2) There are non-atomic stores to service_mask. Is it correct that the stores
> themselves aren't the issue, but relative visibility of service_mask stores vs
> num_mapped_cores? (Detail in (3) below)
> 
> 
> >  		if (*set && !lcore_mapped) {
> >  			lcore_states[lcore].service_mask |= sid_mask;
> > -
> 	rte_atomic32_inc(&rte_services[sid].num_mapped_cores);
> > +
> 	__atomic_add_fetch(&rte_services[sid].num_mapped_cores,
> > +					    1, __ATOMIC_RELEASE);
> >  		}
> >  		if (!*set && lcore_mapped) {
> >  			lcore_states[lcore].service_mask &= ~(sid_mask);
> > -
> 	rte_atomic32_dec(&rte_services[sid].num_mapped_cores);
> > +
> 	__atomic_sub_fetch(&rte_services[sid].num_mapped_cores,
> > +					    1, __ATOMIC_RELEASE);
> >  		}
> 
> 3) Here we update the core-local service_mask, and then update the
> num_mapped_cores with an ATOMIC_RELEASE. The RELEASE here ensures
> that the previous store to service_mask is guaranteed to be visible on all cores
> if this store is visible. Why do we care about this property?
> The service_mask is core local anway.
We are working on concurrency between the reader and writer. The service_mask is local to the core, but it is accessed by a reader and writer.
I think we should wait to conclude on the meaning of 'num_mapped_cores', that will dictate what the order should be. For ex: if it is just for statistics purpose, then we could use just RELAXED memory order and then the order for service_mask will also change.

> 
> 4) Even with the load ACQ service_mask, and REL num_mapped_cores store,
> is there not still a race-condition possible where 2 lcores simultaneously load-
> ACQ the service_mask, and then both do atomic add/sub_fetch with REL?
> 
> 5) Assuming 4 above race is true, it raises the real question - the service-cores
> control APIs are not designed to be multi-thread-safe. Orchestration of
> service/lcore mappings is not meant to be done by multiple threads at the
> same time. Documenting this loudly may help, I'm happy to send a patch to do
> so if we're agreed on the above?
I completely agree here. writer-writer concurrency is another topic and we should (for now at least) say that the control plane APIs are not thread safe.

> 
> 
> 
> 
> >  	}
> >
> >  	if (enabled)
> >  		*enabled = !!(lcore_states[lcore].service_mask & (sid_mask));
> >
> > -	rte_smp_wmb();
> > -
> >  	return 0;
> >  }
> >
> > @@ -625,7 +643,8 @@ rte_service_lcore_reset_all(void)
> >  		}
> >  	}
> >  	for (i = 0; i < RTE_SERVICE_NUM_MAX; i++)
> > -		rte_atomic32_set(&rte_services[i].num_mapped_cores, 0);
> > +		__atomic_store_n(&rte_services[i].num_mapped_cores, 0,
> > +				    __ATOMIC_RELAXED);
> >
> >  	rte_smp_wmb();
> >
> > @@ -708,7 +727,8 @@ rte_service_lcore_stop(uint32_t lcore)
> >  		int32_t enabled = service_mask & (UINT64_C(1) << i);
> >  		int32_t service_running = rte_service_runstate_get(i);
> >  		int32_t only_core = (1 ==
> > -
> 	rte_atomic32_read(&rte_services[i].num_mapped_cores));
> > +
> 	__atomic_load_n(&rte_services[i].num_mapped_cores,
> > +					__ATOMIC_RELAXED));
> >
> >  		/* if the core is mapped, and the service is running, and this
> >  		 * is the only core that is mapped, the service would cease to
> > --
> > 2.7.4