Re: [dpdk-dev] [PATCH 2/2] lib/timer: relax barrier for status update

["Carrillo, Erik G" <erik.g.carrillo@intel.com>]

> -----Original Message-----
> From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Sent: Wednesday, April 8, 2020 4:16 PM
> To: Carrillo, Erik G <erik.g.carrillo@intel.com>; Phil Yang
> <Phil.Yang@arm.com>; rsanford@akamai.com; dev@dpdk.org
> Cc: david.marchand@redhat.com; Burakov, Anatoly
> <anatoly.burakov@intel.com>; thomas@monjalon.net; jerinj@marvell.com;
> hemant.agrawal@nxp.com; Gavin Hu <Gavin.Hu@arm.com>; nd
> <nd@arm.com>; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> nd <nd@arm.com>
> Subject: RE: [PATCH 2/2] lib/timer: relax barrier for status update
> 
> <snip>
> 
> > > Subject: [PATCH 2/2] lib/timer: relax barrier for status update
> > >
> > > Volatile has no ordering semantics. The rte_timer structure defines
> > > timer status as a volatile variable and uses the rte_r/wmb barrier
> > > to guarantee inter-thread visibility.
> > >
> > > This patch optimized the volatile operation with c11 atomic
> > > operations and one-way barrier to save the performance penalty.
> > > According to the timer_perf_autotest benchmarking results, this
> > > patch can uplift 10%~16% timer appending performance, 3%~20% timer
> > > resetting performance and 45% timer callbacks scheduling performance
> > > on aarch64 and no loss in performance for x86.
> > >
> > > Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> >
> > Hi Phil,
> >
> > It seems like the consensus is to generally avoid replacing rte_atomic_*
> > interfaces with the GCC builtins directly.   In other areas of DPDK that are
> > being patched, are the <std_atomic.h> C11 APIs going to be investigated?
> It
> > seems like that decision will apply here as well.
> Agree. The new APIs are going to be 1 to 1 mapped with the built-in intrinsics
> (the memory orderings used themselves will not change). We should go
> ahead with the review and conclude any issues. Once the decision is made
> on what APIs to use, we can submit the next version using the APIs decided.
> 
Thanks, Honnappa.

I have reviewed the memory orderings and I see no issues with them.   I do have a question regarding a comment - I'll pose it inline:

> >
> > Thanks,
> > Erik
> >
> > > ---
> > >  lib/librte_timer/rte_timer.c | 90
> > > +++++++++++++++++++++++++++++++----
> > > ---------
> > >  lib/librte_timer/rte_timer.h |  2 +-
> > >  2 files changed, 65 insertions(+), 27 deletions(-)
> > >
> > > diff --git a/lib/librte_timer/rte_timer.c
> > > b/lib/librte_timer/rte_timer.c index 269e921..be0262d 100644
> > > --- a/lib/librte_timer/rte_timer.c
> > > +++ b/lib/librte_timer/rte_timer.c
> > > @@ -10,7 +10,6 @@
> > >  #include <assert.h>
> > >  #include <sys/queue.h>
> > >
> > > -#include <rte_atomic.h>
> > >  #include <rte_common.h>
> > >  #include <rte_cycles.h>
> > >  #include <rte_eal_memconfig.h>
> > > @@ -218,7 +217,7 @@ rte_timer_init(struct rte_timer *tim)
> > >
> > >  	status.state = RTE_TIMER_STOP;
> > >  	status.owner = RTE_TIMER_NO_OWNER;
> > > -	tim->status.u32 = status.u32;
> > > +	__atomic_store_n(&tim->status.u32, status.u32,
> > > __ATOMIC_RELAXED);
> > >  }
> > >
> > >  /*
> > > @@ -239,9 +238,9 @@ timer_set_config_state(struct rte_timer *tim,
> > >
> > >  	/* wait that the timer is in correct status before update,
> > >  	 * and mark it as being configured */
> > > -	while (success == 0) {
> > > -		prev_status.u32 = tim->status.u32;
> > > +	prev_status.u32 = __atomic_load_n(&tim->status.u32,
> > > __ATOMIC_RELAXED);
> > >
> > > +	while (success == 0) {
> > >  		/* timer is running on another core
> > >  		 * or ready to run on local core, exit
> > >  		 */
> > > @@ -258,9 +257,20 @@ timer_set_config_state(struct rte_timer *tim,
> > >  		 * mark it atomically as being configured */
> > >  		status.state = RTE_TIMER_CONFIG;
> > >  		status.owner = (int16_t)lcore_id;
> > > -		success = rte_atomic32_cmpset(&tim->status.u32,
> > > -					      prev_status.u32,
> > > -					      status.u32);
> > > +		/* If status is observed as RTE_TIMER_CONFIG earlier,
> > > +		 * that's not going to cause any issues because the
> > > +		 * pattern is read for status then read the other members.

I don't follow the above comment.  What is meant by "earlier"?

Thanks,
Erik

> > > +		 * In one of the callers to timer_set_config_state
> > > +		 * (the __rte_timer_reset) we set other members to the
> > > +		 * structure (period, expire, f, arg) we want these
> > > +		 * changes to be observed after our change to status.
> > > +		 * So we need __ATOMIC_ACQUIRE here.
> > > +		 */
> > > +		success = __atomic_compare_exchange_n(&tim-
> > > >status.u32,
> > > +					      &prev_status.u32,
> > > +					      status.u32, 0,
> > > +					      __ATOMIC_ACQUIRE,
> > > +					      __ATOMIC_RELAXED);
> > >  	}
> > >
> > >  	ret_prev_status->u32 = prev_status.u32; @@ -279,20 +289,27 @@
> > > timer_set_running_state(struct rte_timer *tim)
> > >
> > >  	/* wait that the timer is in correct status before update,
> > >  	 * and mark it as running */
> > > -	while (success == 0) {
> > > -		prev_status.u32 = tim->status.u32;
> > > +	prev_status.u32 = __atomic_load_n(&tim->status.u32,
> > > __ATOMIC_RELAXED);
> > >
> > > +	while (success == 0) {
> > >  		/* timer is not pending anymore */
> > >  		if (prev_status.state != RTE_TIMER_PENDING)
> > >  			return -1;
> > >
> > >  		/* here, we know that timer is stopped or pending,
> > > -		 * mark it atomically as being configured */
> > > +		 * mark it atomically as being running
> > > +		 */
> > >  		status.state = RTE_TIMER_RUNNING;
> > >  		status.owner = (int16_t)lcore_id;
> > > -		success = rte_atomic32_cmpset(&tim->status.u32,
> > > -					      prev_status.u32,
> > > -					      status.u32);
> > > +		/* RUNNING states are acting as locked states. If the
> > > +		 * timer is in RUNNING state, the state cannot be changed
> > > +		 * by other threads. So, we should use ACQUIRE here.
> > > +		 */
> > > +		success = __atomic_compare_exchange_n(&tim-
> > > >status.u32,
> > > +					      &prev_status.u32,
> > > +					      status.u32, 0,
> > > +					      __ATOMIC_ACQUIRE,
> > > +					      __ATOMIC_RELAXED);
> > >  	}
> > >
> > >  	return 0;
> > > @@ -520,10 +537,12 @@ __rte_timer_reset(struct rte_timer *tim,
> > > uint64_t expire,
> > >
> > >  	/* update state: as we are in CONFIG state, only us can modify
> > >  	 * the state so we don't need to use cmpset() here */
> > > -	rte_wmb();
> > >  	status.state = RTE_TIMER_PENDING;
> > >  	status.owner = (int16_t)tim_lcore;
> > > -	tim->status.u32 = status.u32;
> > > +	/* The "RELEASE" ordering guarantees the memory operations above
> > > +	 * the status update are observed before the update by all threads
> > > +	 */
> > > +	__atomic_store_n(&tim->status.u32, status.u32,
> > > __ATOMIC_RELEASE);
> > >
> > >  	if (tim_lcore != lcore_id || !local_is_locked)
> > >  		rte_spinlock_unlock(&priv_timer[tim_lcore].list_lock);
> > > @@ -600,10 +619,12 @@ __rte_timer_stop(struct rte_timer *tim, int
> > > local_is_locked,
> > >  	}
> > >
> > >  	/* mark timer as stopped */
> > > -	rte_wmb();
> > >  	status.state = RTE_TIMER_STOP;
> > >  	status.owner = RTE_TIMER_NO_OWNER;
> > > -	tim->status.u32 = status.u32;
> > > +	/* The "RELEASE" ordering guarantees the memory operations above
> > > +	 * the status update are observed before the update by all threads
> > > +	 */
> > > +	__atomic_store_n(&tim->status.u32, status.u32,
> > > __ATOMIC_RELEASE);
> > >
> > >  	return 0;
> > >  }
> > > @@ -637,7 +658,8 @@ rte_timer_stop_sync(struct rte_timer *tim)  int
> > > rte_timer_pending(struct rte_timer *tim)  {
> > > -	return tim->status.state == RTE_TIMER_PENDING;
> > > +	return __atomic_load_n(&tim->status.state,
> > > +				__ATOMIC_RELAXED) ==
> > > RTE_TIMER_PENDING;
> > >  }
> > >
> > >  /* must be called periodically, run all timer that expired */ @@
> > > -739,8
> > > +761,12 @@ __rte_timer_manage(struct rte_timer_data *timer_data)
> > >  			/* remove from done list and mark timer as stopped
> > */
> > >  			status.state = RTE_TIMER_STOP;
> > >  			status.owner = RTE_TIMER_NO_OWNER;
> > > -			rte_wmb();
> > > -			tim->status.u32 = status.u32;
> > > +			/* The "RELEASE" ordering guarantees the memory
> > > +			 * operations above the status update are observed
> > > +			 * before the update by all threads
> > > +			 */
> > > +			__atomic_store_n(&tim->status.u32, status.u32,
> > > +				__ATOMIC_RELEASE);
> > >  		}
> > >  		else {
> > >  			/* keep it in list and mark timer as pending */ @@ -
> > > 748,8 +774,12 @@ __rte_timer_manage(struct rte_timer_data
> *timer_data)
> > >  			status.state = RTE_TIMER_PENDING;
> > >  			__TIMER_STAT_ADD(priv_timer, pending, 1);
> > >  			status.owner = (int16_t)lcore_id;
> > > -			rte_wmb();
> > > -			tim->status.u32 = status.u32;
> > > +			/* The "RELEASE" ordering guarantees the memory
> > > +			 * operations above the status update are observed
> > > +			 * before the update by all threads
> > > +			 */
> > > +			__atomic_store_n(&tim->status.u32, status.u32,
> > > +				__ATOMIC_RELEASE);
> > >  			__rte_timer_reset(tim, tim->expire + tim->period,
> > >  				tim->period, lcore_id, tim->f, tim->arg, 1,
> > >  				timer_data);
> > > @@ -919,8 +949,12 @@ rte_timer_alt_manage(uint32_t timer_data_id,
> > >  			/* remove from done list and mark timer as stopped
> > */
> > >  			status.state = RTE_TIMER_STOP;
> > >  			status.owner = RTE_TIMER_NO_OWNER;
> > > -			rte_wmb();
> > > -			tim->status.u32 = status.u32;
> > > +			/* The "RELEASE" ordering guarantees the memory
> > > +			 * operations above the status update are observed
> > > +			 * before the update by all threads
> > > +			 */
> > > +			__atomic_store_n(&tim->status.u32, status.u32,
> > > +				__ATOMIC_RELEASE);
> > >  		} else {
> > >  			/* keep it in list and mark timer as pending */
> > >  			rte_spinlock_lock(
> > > @@ -928,8 +962,12 @@ rte_timer_alt_manage(uint32_t timer_data_id,
> > >  			status.state = RTE_TIMER_PENDING;
> > >  			__TIMER_STAT_ADD(data->priv_timer, pending, 1);
> > >  			status.owner = (int16_t)this_lcore;
> > > -			rte_wmb();
> > > -			tim->status.u32 = status.u32;
> > > +			/* The "RELEASE" ordering guarantees the memory
> > > +			 * operations above the status update are observed
> > > +			 * before the update by all threads
> > > +			 */
> > > +			__atomic_store_n(&tim->status.u32, status.u32,
> > > +				__ATOMIC_RELEASE);
> > >  			__rte_timer_reset(tim, tim->expire + tim->period,
> > >  				tim->period, this_lcore, tim->f, tim->arg, 1,
> > >  				data);
> > > diff --git a/lib/librte_timer/rte_timer.h
> > > b/lib/librte_timer/rte_timer.h index c6b3d45..df533fa 100644
> > > --- a/lib/librte_timer/rte_timer.h
> > > +++ b/lib/librte_timer/rte_timer.h
> > > @@ -101,7 +101,7 @@ struct rte_timer  {
> > >  	uint64_t expire;       /**< Time when timer expire. */
> > >  	struct rte_timer *sl_next[MAX_SKIPLIST_DEPTH];
> > > -	volatile union rte_timer_status status; /**< Status of timer. */
> > > +	union rte_timer_status status; /**< Status of timer. */
> > >  	uint64_t period;       /**< Period of timer (0 if not periodic). */
> > >  	rte_timer_cb_t f;      /**< Callback function. */
> > >  	void *arg;             /**< Argument to callback function. */
> > > --
> > > 2.7.4