Re: [dpdk-dev] [PATCH v6 1/2] eal/ticketlock: ticket based to improve fairness

["Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>]

Hi Konstantin,

> -----Original Message-----
> From: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Sent: Tuesday, March 19, 2019 6:15 PM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>; dev@dpdk.org
> Cc: nd <nd@arm.com>; stephen@networkplumber.org;
> jerin.jacob@caviumnetworks.com; thomas@monjalon.net; Honnappa
> Nagarahalli <Honnappa.Nagarahalli@arm.com>; Joyce Kong (Arm Technology
> China) <Joyce.Kong@arm.com>
> Subject: RE: [dpdk-dev] [PATCH v6 1/2] eal/ticketlock: ticket based to improve
> fairness
> 
> 
> Hi Gavin,
> 
> > >
> > > > diff --git a/lib/librte_eal/common/include/generic/rte_ticketlock.h
> > > b/lib/librte_eal/common/include/generic/rte_ticketlock.h
> > > > new file mode 100644
> > > > index 0000000..d63aaaa
> > > > --- /dev/null
> > > > +++ b/lib/librte_eal/common/include/generic/rte_ticketlock.h
> > > > @@ -0,0 +1,308 @@
> > > > +/* SPDX-License-Identifier: BSD-3-Clause
> > > > + * Copyright(c) 2019 Arm Limited
> > > > + */
> > > > +
> > > > +#ifndef _RTE_TICKETLOCK_H_
> > > > +#define _RTE_TICKETLOCK_H_
> > > > +
> > > > +/**
> > > > + * @file
> > > > + *
> > > > + * RTE ticket locks
> > > > + *
> > > > + * This file defines an API for ticket locks, which give each waiting
> > > > + * thread a ticket and take the lock one by one, first come, first
> > > > + * serviced.
> > > > + *
> > > > + * All locks must be initialised before use, and only initialised once.
> > > > + *
> > > > + */
> > > > +
> > > > +#ifdef __cplusplus
> > > > +extern "C" {
> > > > +#endif
> > > > +
> > > > +#include <rte_common.h>
> > > > +#include <rte_lcore.h>
> > > > +#include <rte_pause.h>
> > > > +
> > > > +/**
> > > > + * The rte_ticketlock_t type.
> > > > + */
> > > > +typedef struct {
> > > > +	uint16_t current;
> > > > +	uint16_t next;
> > > > +} rte_ticketlock_t;
> > > > +
> > > > +/**
> > > > + * A static ticketlock initializer.
> > > > + */
> > > > +#define RTE_TICKETLOCK_INITIALIZER { 0 }
> > > > +
> > > > +/**
> > > > + * Initialize the ticketlock to an unlocked state.
> > > > + *
> > > > + * @param tl
> > > > + *   A pointer to the ticketlock.
> > > > + */
> > > > +static inline __rte_experimental void
> > > > +rte_ticketlock_init(rte_ticketlock_t *tl)
> > > > +{
> > > > +	__atomic_store_n(&tl->current, 0, __ATOMIC_RELAXED);
> > > > +	__atomic_store_n(&tl->next, 0, __ATOMIC_RELAXED);
> > > > +}
> > > > +
> > > > +/**
> > > > + * Take the ticketlock.
> > > > + *
> > > > + * @param tl
> > > > + *   A pointer to the ticketlock.
> > > > + */
> > > > +static inline __rte_experimental void
> > > > +rte_ticketlock_lock(rte_ticketlock_t *tl)
> > > > +{
> > > > +	uint16_t me = __atomic_fetch_add(&tl->next, 1,
> > > __ATOMIC_RELAXED);
> > > > +	while (__atomic_load_n(&tl->current, __ATOMIC_ACQUIRE) != me)
> > > > +		rte_pause();
> > > > +}
> > > > +
> > > > +/**
> > > > + * Release the ticketlock.
> > > > + *
> > > > + * @param tl
> > > > + *   A pointer to the ticketlock.
> > > > + */
> > > > +static inline __rte_experimental void
> > > > +rte_ticketlock_unlock(rte_ticketlock_t *tl)
> > > > +{
> > > > +	uint16_t i = __atomic_load_n(&tl->current, __ATOMIC_RELAXED);
> > > > +	__atomic_store_n(&tl->current, i+1, __ATOMIC_RELEASE);
> > > > +}
> > > > +
> > > > +/**
> > > > + * Try to take the lock.
> > > > + *
> > > > + * @param tl
> > > > + *   A pointer to the ticketlock.
> > > > + * @return
> > > > + *   1 if the lock is successfully taken; 0 otherwise.
> > > > + */
> > > > +static inline __rte_experimental int
> > > > +rte_ticketlock_trylock(rte_ticketlock_t *tl)
> > > > +{
> > > > +	uint16_t next = __atomic_load_n(&tl->next, __ATOMIC_RELAXED);
> > > > +	uint16_t cur = __atomic_load_n(&tl->current, __ATOMIC_RELAXED);
> > > > +	if (next == cur) {
> > >
> > > Probably a naïve one:
> > > Suppose next==cur==1 here, then this thread will experience really long
> > > context switch,
> >
> > By saying context switch, do you mean running to here, it is out of CPU time
> and starving for CPU?
> 
> Yes.
> 
> >
> > > so next time it continues its execution tl->next value will wrap-up and will
> > > be 1 again, and tl->current==0 (lock held).
> > > I suppose this function will set tl->next=2 and will return a success?
> >
> > If this thread was swapped out and another thread took/attempted to take
> the lock, yes, tl->next == 2 here,
> > But as next == 1 unchanged, so it would not return a success.
> 
> I am not talking about situation when tl->next == 2,tl->current==1 (just one
> lock() was executed by different thread).
> I am talking about situation when this thread was out of cpu for significant
> amount of cycles,
> and in that period tl->next and tl->current were wrapped around (they both
> reached UINT16_MAX, then 0).
> i.e. UINT16_MAX lock/unlock were executed while this thread was away from
> cpu.
> After that another thread just did successful lock(), so tl->next==1 and tl-
> >current==0.
> Now this thread wakeups and continues with:
> __atomic_compare_exchange_n(&tl->next, &next, next+1, ...)
> As both tl->next==1 and next==1, it will succeed.
> So we have 2 threads assuming they grabbed the lock successfully.
> Konstantin
> 
Now I understood your points, but not sure if it is a rare or even impossible case for this thread stalls for CPU and during this time, the other threads have taken the lock for 2^16 times, to wrap up. 

Anyway I made a patch, currently in internal review to fix this issue, the basic idea is to compare not only the next, but also the current, and update the next(+1 and take the lock) only if both of them were not changed(or wrapped up and the lock released).
I will submit the patch after internal review approved. Please let me know if you have more comments.

> >
> > > Wouldn't be better here and in _is_locked_ to do load/store for
> > > next/current values in one go
> > > (using 32bit op)?
> > > Konstantin
> >
> > To load both in one go is feasible, but no necessary as we need to compare
> them.
> > We don't need store both as in this function tl->current is read only.
> > tl->next is read-update-store, I ever thought of combining the two if-
> statements to one __atomic_compare_exchange_n(&(&tl->next,&tl-
> > >current, tl->next+1, ...),
> > but tl->next+1 is out of atomicity and may be the old value and corrupt the
> ticket lock waiting chain.
> >
> > The current code works ok except it may fail spuriously(in case during
> context switch, the lock was taken and released by other threads,
> > moving tl->next forward, in this case
> > The lock is available but not obtained by this trylock).  Anyway, as the name
> suggests, it is a try/attempt, a spurious fail is not a big deal?
> > And in most cases, dpdk running on dedicated cores,
> > the context switch will not happen at all.
> >
> > Any more comments are welcome!
> > >
> > > > +		if (__atomic_compare_exchange_n(&tl->next, &next,
> > > next+1,
> > > > +		    0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
> > > > +			return 1;
> > > > +	}
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +/**
> > > > + * Test if the lock is taken.
> > > > + *
> > > > + * @param tl
> > > > + *   A pointer to the ticketlock.
> > > > + * @return
> > > > + *   1 if the lock icurrently taken; 0 otherwise.
> > > > + */
> > > > +static inline __rte_experimental int
> > > > +rte_ticketlock_is_locked(rte_ticketlock_t *tl)
> > > > +{
> > > > +	return (__atomic_load_n(&tl->current, __ATOMIC_ACQUIRE) !=
> > > > +		__atomic_load_n(&tl->next, __ATOMIC_ACQUIRE));
> > > > +}
> > > > +