Re: [RFC git tree] Userspace RCU (urcu) for Linux (repost)

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Feb 08, 2009 at 07:54:50PM -0500, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Sun, Feb 08, 2009 at 02:36:06PM -0800, Paul E. McKenney wrote:
> > > On Sun, Feb 08, 2009 at 04:46:10PM -0500, Mathieu Desnoyers wrote:
> > 
> > [ . . . ]
> > 
> > > > I ran your modified version within my benchmarks :
> > > > 
> > > > with return value : 14.164 cycles per read
> > > > without return value : 16.4017 cycles per read
> > > > 
> > > > So we have a 14% performance decrease due to this. We also pollute the
> > > > branch prediction buffer and we add a cache access due to the added
> > > > variables in the TLS. Returning the value has the clear advantage of
> > > > letting the compiler keep it around in registers or on the stack, which
> > > > clearly costs less.
> > > > 
> > > > So I think the speed factor outweights the visual considerations. Maybe
> > > > we could switch to something like :
> > > > 
> > > > unsigned int qparity;
> > > > 
> > > > urcu_read_lock(&qparity);
> > > > ...
> > > > urcu_read_unlock(&qparity);
> > > > 
> > > > That would be a bit like local_irq_save() in the kernel, except that we
> > > > could do it in a static inline because we pass the address. I
> > > > personnally dislike the local_irq_save() way of hiding the fact that it
> > > > writes to the variable in a "clever" macro. I'd really prefer to leave
> > > > the " & ".
> > > > 
> > > > What is your opinion ?
> > > 
> > > My current opinion is that I can avoid the overflow problem and the
> > > need to recheck, which might get rid of the need for both arguments
> > > and return values while still maintaining good performance.  The trick
> > > is to use only the topmost bit for the grace-period counter, and all
> > > the rest of the bits for nesting.  That way, no matter what value of
> > > global counter one picks up, it will be waited for (since there are but
> > > two values that the global counter takes on).
> > > 
> > > But just now coding it, so will see if it actually works.
> > 
> > Seems to work, and seems to be pretty fast on my machine, anyway.
> > This one adapts itself to 32- and 64-bit machines, though almost
> > all of the code is common.  It does do a check, but avoids array
> > indexing, arguments, and return values.
> > 
> > How does it do on your hardware?
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Wow...
> 
> Patch updated against HEAD.
> 
> Time per read : 7.53622 cycles
> 
> Half of what we had previously.. I'll have to look at the assembly. :)

My guess is that CMOV is your friend here...  ;-)

							Thanx, Paul

> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> ---
> 
>  test_urcu.c        |    6 +++---
>  test_urcu_timing.c |    6 +++---
>  urcu.c             |   23 ++++++++++-------------
>  urcu.h             |   42 +++++++++++++++++++++++++++++-------------
>  4 files changed, 45 insertions(+), 32 deletions(-)
> 
> diff --git a/test_urcu.c b/test_urcu.c
> index f6be45b..f115a4a 100644
> --- a/test_urcu.c
> +++ b/test_urcu.c
> @@ -72,7 +72,7 @@ void rcu_copy_mutex_unlock(void)
> 
>  void *thr_reader(void *arg)
>  {
> -	int qparity, i, j;
> +	int i, j;
>  	struct test_array *local_ptr;
> 
>  	printf("thread %s, thread id : %lx, tid %lu\n",
> @@ -83,14 +83,14 @@ void *thr_reader(void *arg)
> 
>  	for (i = 0; i < 100000; i++) {
>  		for (j = 0; j < 100000000; j++) {
> -			rcu_read_lock(&qparity);
> +			rcu_read_lock();
>  			local_ptr = rcu_dereference(test_rcu_pointer);
>  			if (local_ptr) {
>  				assert(local_ptr->a == 8);
>  				assert(local_ptr->b == 12);
>  				assert(local_ptr->c[55] == 2);
>  			}
> -			rcu_read_unlock(&qparity);
> +			rcu_read_unlock();
>  		}
>  	}
> 
> diff --git a/test_urcu_timing.c b/test_urcu_timing.c
> index 57fda4f..9903705 100644
> --- a/test_urcu_timing.c
> +++ b/test_urcu_timing.c
> @@ -94,7 +94,7 @@ static cycles_t reader_time[NR_READ] __attribute__((aligned(128)));
> 
>  void *thr_reader(void *arg)
>  {
> -	int qparity, i, j;
> +	int i, j;
>  	struct test_array *local_ptr;
>  	cycles_t time1, time2;
> 
> @@ -107,12 +107,12 @@ void *thr_reader(void *arg)
>  	time1 = get_cycles();
>  	for (i = 0; i < OUTER_READ_LOOP; i++) {
>  		for (j = 0; j < INNER_READ_LOOP; j++) {
> -			rcu_read_lock(&qparity);
> +			rcu_read_lock();
>  			local_ptr = rcu_dereference(test_rcu_pointer);
>  			if (local_ptr) {
>  				assert(local_ptr->a == 8);
>  			}
> -			rcu_read_unlock(&qparity);
> +			rcu_read_unlock();
>  		}
>  	}
>  	time2 = get_cycles();
> diff --git a/urcu.c b/urcu.c
> index 08fb75d..2914b66 100644
> --- a/urcu.c
> +++ b/urcu.c
> @@ -19,17 +19,17 @@
> 
>  pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
> 
> -/* Global quiescent period parity */
> -int urcu_qparity;
> +/* Global grace period counter */
> +long urcu_gp_ctr;
> 
> -int __thread urcu_active_readers[2];
> +long __thread urcu_active_readers;
> 
>  /* Thread IDs of registered readers */
>  #define INIT_NUM_THREADS 4
> 
>  struct reader_data {
>  	pthread_t tid;
> -	int *urcu_active_readers;
> +	long *urcu_active_readers;
>  };
> 
>  static struct reader_data *reader_data;
> @@ -60,11 +60,9 @@ void internal_urcu_unlock(void)
>  /*
>   * called with urcu_mutex held.
>   */
> -static int switch_next_urcu_qparity(void)
> +static void switch_next_urcu_qparity(void)
>  {
> -	int old_parity = urcu_qparity;
> -	urcu_qparity = 1 - old_parity;
> -	return old_parity;
> +	urcu_gp_ctr += RCU_GP_CTR_BOTTOM_BIT;
>  }
> 
>  static void force_mb_all_threads(void)
> @@ -89,7 +87,7 @@ static void force_mb_all_threads(void)
>  	mb();	/* read sig_done before ending the barrier */
>  }
> 
> -void wait_for_quiescent_state(int parity)
> +void wait_for_quiescent_state(void)
>  {
>  	struct reader_data *index;
> 
> @@ -101,7 +99,7 @@ void wait_for_quiescent_state(int parity)
>  		/*
>  		 * BUSY-LOOP.
>  		 */
> -		while (index->urcu_active_readers[parity] != 0)
> +		while (rcu_old_gp_ongoing(index->urcu_active_readers))
>  			barrier();
>  	}
>  	/*
> @@ -115,17 +113,16 @@ void wait_for_quiescent_state(int parity)
> 
>  static void switch_qparity(void)
>  {
> -	int prev_parity;
> 
>  	/* All threads should read qparity before accessing data structure. */
>  	/* Write ptr before changing the qparity */
>  	force_mb_all_threads();
> -	prev_parity = switch_next_urcu_qparity();
> +	switch_next_urcu_qparity();
> 
>  	/*
>  	 * Wait for previous parity to be empty of readers.
>  	 */
> -	wait_for_quiescent_state(prev_parity);
> +	wait_for_quiescent_state();
>  }
> 
>  void synchronize_rcu(void)
> diff --git a/urcu.h b/urcu.h
> index b6b5c7b..e83c69f 100644
> --- a/urcu.h
> +++ b/urcu.h
> @@ -66,23 +66,39 @@ static inline void atomic_inc(int *v)
> 
>  #define SIGURCU SIGUSR1
> 
> -/* Global quiescent period parity */
> -extern int urcu_qparity;
> +#define RCU_GP_CTR_BOTTOM_BIT (sizeof(long) == 4 ? 0x80000000 : 0x100L)
> +#define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BOTTOM_BIT - 1)
> 
> -extern int __thread urcu_active_readers[2];
> +/* Global quiescent period counter with low-order bits unused. */
> +extern long urcu_gp_ctr;
> 
> -static inline int get_urcu_qparity(void)
> +extern long __thread urcu_active_readers;
> +
> +static inline int rcu_old_gp_ongoing(long *value)
>  {
> -	return urcu_qparity;
> +	long v;
> +
> +	if (value == NULL)
> +		return 0;
> +	v = ACCESS_ONCE(*value);
> +	if (sizeof(long) == 4) {
> +		return (v & RCU_GP_CTR_NEST_MASK) &&
> +		       ((v ^ ACCESS_ONCE(urcu_gp_ctr)) & ~RCU_GP_CTR_NEST_MASK);
> +	} else {
> +		return (v & RCU_GP_CTR_NEST_MASK) &&
> +		       (v - ACCESS_ONCE(urcu_gp_ctr) < 0);
> +	}
>  }
> 
> -/*
> - * urcu_parity should be declared on the caller's stack.
> - */
> -static inline void rcu_read_lock(int *urcu_parity)
> +static inline void rcu_read_lock(void)
>  {
> -	*urcu_parity = get_urcu_qparity();
> -	urcu_active_readers[*urcu_parity]++;
> +	long tmp;
> +
> +	tmp = urcu_active_readers;
> +	if ((tmp & RCU_GP_CTR_NEST_MASK) == 0)
> +		urcu_active_readers = urcu_gp_ctr + 1;
> +	else
> +		urcu_active_readers = tmp + 1;
>  	/*
>  	 * Increment active readers count before accessing the pointer.
>  	 * See force_mb_all_threads().
> @@ -90,14 +106,14 @@ static inline void rcu_read_lock(int *urcu_parity)
>  	barrier();
>  }
> 
> -static inline void rcu_read_unlock(int *urcu_parity)
> +static inline void rcu_read_unlock(void)
>  {
>  	barrier();
>  	/*
>  	 * Finish using rcu before decrementing the pointer.
>  	 * See force_mb_all_threads().
>  	 */
> -	urcu_active_readers[*urcu_parity]--;
> +	urcu_active_readers--;
>  }
> 
>  extern void *urcu_publish_content(void **ptr, void *new);
> 
> 
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68