[RFC] ring: further performance improvements with C11

[Wathsala Wathawana Vithanage]

> -----Original Message-----
> From: Konstantin Ananyev <konstantin.ananyev at huawei.com>
> Sent: Wednesday, August 2, 2023 4:43 AM
> To: Wathsala Wathawana Vithanage <wathsala.vithanage at arm.com>;
> Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>;
> konstantin.v.ananyev at yandex.ru; thomas at monjalon.net; Ruifeng Wang
> <Ruifeng.Wang at arm.com>
> Cc: dev at dpdk.org; nd <nd at arm.com>; Justin He <Justin.He at arm.com>
> Subject: RE: [RFC] ring: further performance improvements with C11
> 
> 
> 
> > For improved performance over the current C11 based ring
> > implementation following changes were made.
> > (1) Replace tail store with RELEASE semantics in
> > __rte_ring_update_tail with a RELEASE fence. Replace load of the tail
> > with ACQUIRE semantics in __rte_ring_move_prod_head and
> > __rte_ring_move_cons_head with ACQUIRE fences.
> > (2) Remove ACQUIRE fences between load of the old_head and load of the
> > cons_tail in __rte_ring_move_prod_head and __rte_ring_move_cons_head.
> > These two fences are not required for the safety of the ring library.
> 
> Hmm... with these changes, aren't we re-introducing the old bug fixed by this
> commit:

Cover letter explains why this barrier does not solve what it intends to solve and
Why it should not matter.
https://mails.dpdk.org/archives/dev/2023-June/270874.html

> 
> commit 9bc2cbb007c0a3335c5582357ae9f6d37ea0b654
> Author: Jia He <justin.he at arm.com>
> Date:   Fri Nov 10 03:30:42 2017 +0000
> 
>     ring: guarantee load/load order in enqueue and dequeue
> 
>     We watched a rte panic of mbuf_autotest in our qualcomm arm64 server
>     (Amberwing).
> 
>     Root cause:
>     In __rte_ring_move_cons_head()
>     ...
>             do {
>                     /* Restore n as it may change every loop */
>                     n = max;
> 
>                     *old_head = r->cons.head;                //1st load
>                     const uint32_t prod_tail = r->prod.tail; //2nd load
> 
>     In weak memory order architectures (powerpc,arm), the 2nd load might be
>     reodered before the 1st load, that makes *entries is bigger than we wanted.
>     This nasty reording messed enque/deque up.
>     ....
> ?
> 
> >
> > Signed-off-by: Wathsala Vithanage <wathsala.vithanage at arm.com>
> > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
> > Reviewed-by: Ruifeng Wang <ruifeng.wang at arm.com>
> > ---
> >  .mailmap                    |  1 +
> >  lib/ring/rte_ring_c11_pvt.h | 35 ++++++++++++++++++++---------------
> >  2 files changed, 21 insertions(+), 15 deletions(-)
> >
> > diff --git a/.mailmap b/.mailmap
> > index 4018f0fc47..367115d134 100644
> > --- a/.mailmap
> > +++ b/.mailmap
> > @@ -1430,6 +1430,7 @@ Walter Heymans
> <walter.heymans at corigine.com>
> > Wang Sheng-Hui <shhuiw at gmail.com>  Wangyu (Eric)
> > <seven.wangyu at huawei.com>  Waterman Cao <waterman.cao at intel.com>
> > +Wathsala Vithanage <wathsala.vithanage at arm.com>
> >  Weichun Chen <weichunx.chen at intel.com>  Wei Dai <wei.dai at intel.com>
> > Weifeng Li <liweifeng96 at 126.com> diff --git
> > a/lib/ring/rte_ring_c11_pvt.h b/lib/ring/rte_ring_c11_pvt.h index
> > f895950df4..63fe58ce9e 100644
> > --- a/lib/ring/rte_ring_c11_pvt.h
> > +++ b/lib/ring/rte_ring_c11_pvt.h
> > @@ -16,6 +16,13 @@ __rte_ring_update_tail(struct rte_ring_headtail *ht,
> uint32_t old_val,
> >  		uint32_t new_val, uint32_t single, uint32_t enqueue)  {
> >  	RTE_SET_USED(enqueue);
> > +	/*
> > +	 * Updating of ht->tail cannot happen before elements are added to or
> > +	 * removed from the ring, as it could result in data races between
> > +	 * producer and consumer threads. Therefore we need a release
> > +	 * barrier here.
> > +	 */
> > +	rte_atomic_thread_fence(__ATOMIC_RELEASE);
> >
> >  	/*
> >  	 * If there are other enqueues/dequeues in progress that preceded
> > us, @@ -24,7 +31,7 @@ __rte_ring_update_tail(struct rte_ring_headtail
> *ht, uint32_t old_val,
> >  	if (!single)
> >  		rte_wait_until_equal_32(&ht->tail, old_val,
> __ATOMIC_RELAXED);
> >
> > -	__atomic_store_n(&ht->tail, new_val, __ATOMIC_RELEASE);
> > +	__atomic_store_n(&ht->tail, new_val, __ATOMIC_RELAXED);
> >  }
> >
> >  /**
> > @@ -66,14 +73,8 @@ __rte_ring_move_prod_head(struct rte_ring *r,
> unsigned int is_sp,
> >  		/* Reset n to the initial burst count */
> >  		n = max;
> >
> > -		/* Ensure the head is read before tail */
> > -		__atomic_thread_fence(__ATOMIC_ACQUIRE);
> > -
> > -		/* load-acquire synchronize with store-release of ht->tail
> > -		 * in update_tail.
> > -		 */
> >  		cons_tail = __atomic_load_n(&r->cons.tail,
> > -					__ATOMIC_ACQUIRE);
> > +					__ATOMIC_RELAXED);
> >
> >  		/* The subtraction is done between two unsigned 32bits value
> >  		 * (the result is always modulo 32 bits even if we have @@ -
> 100,6
> > +101,11 @@ __rte_ring_move_prod_head(struct rte_ring *r, unsigned int
> is_sp,
> >  					0, __ATOMIC_RELAXED,
> >  					__ATOMIC_RELAXED);
> >  	} while (unlikely(success == 0));
> > +	/*
> > +	 * Ensure that updates to the ring doesn't rise above
> > +	 * load of the new_head in SP and MP cases.
> > +	 */
> > +	rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> >  	return n;
> >  }
> >
> > @@ -142,14 +148,8 @@ __rte_ring_move_cons_head(struct rte_ring *r, int
> is_sc,
> >  		/* Restore n as it may change every loop */
> >  		n = max;
> >
> > -		/* Ensure the head is read before tail */
> > -		__atomic_thread_fence(__ATOMIC_ACQUIRE);
> > -
> > -		/* this load-acquire synchronize with store-release of ht->tail
> > -		 * in update_tail.
> > -		 */
> >  		prod_tail = __atomic_load_n(&r->prod.tail,
> > -					__ATOMIC_ACQUIRE);
> > +					__ATOMIC_RELAXED);
> >
> >  		/* The subtraction is done between two unsigned 32bits value
> >  		 * (the result is always modulo 32 bits even if we have @@ -
> 175,6
> > +175,11 @@ __rte_ring_move_cons_head(struct rte_ring *r, int is_sc,
> >  							0,
> __ATOMIC_RELAXED,
> >  							__ATOMIC_RELAXED);
> >  	} while (unlikely(success == 0));
> > +	/*
> > +	 * Ensure that updates to the ring doesn't rise above
> > +	 * load of the new_head in SP and MP cases.
> > +	 */
> > +	rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> >  	return n;
> >  }
> >
> > --
> > 2.25.1
> >