Re: [LTP] [PATCH][real-time] sched_footbal atomic start

[gowrishankar <gomuthuk@linux.vnet.ibm.com>]

Darren Hart wrote:
> The current barrier implementation results in the lowest priority thread
> actually starting the game (they are the last to be scheduled to call
> pthread_barrier_wait).  This thread likely gets a priority boost as it 
> holds
> the hb->lock for the futex associated with the barrier.  This might lead 
> to it
> running ahead of the defense threads.
> In fact, any sort of barrier or cond var implementation (short of a pi 
> aware
> cond broadcast, which is not yet readily available) will result in a 
> thundering
> herd situation when the FUTEX_WAKE_ALL syscall is issued, which can 
> result in
> a short run of one or more offense threads while all the threads get to the
> RUNNABLE state.
> 
> This patch removes the complex starting mechansims and replaces them with a
> simple atomic counter.  All player threads are started and once the
> players_ready count reaches the total player count, the referee starts 
> the game
> by setting the ball position to zero.
> 
> Tested on two platforms (both x86_64, 4 and 8-way) for a combined total of
> 13,000 iterations with 0 failures.
> 
> Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
> 

Acked-by: Gowrishankar <gowrishankar.m@in.ibm.com>

Thanks.
Gowri
> 
> Index: ltp/testcases/realtime/func/sched_football/sched_football.c
> ===================================================================
> --- ltp.orig/testcases/realtime/func/sched_football/sched_football.c    
> 2009-06-24 11:49:16.000000000 -0700
> +++ ltp/testcases/realtime/func/sched_football/sched_football.c    
> 2009-06-24 11:48:44.000000000 -0700
> @@ -47,9 +47,11 @@
>  *
>  * HISTORY
>  *     2006-03-16 Reduced verbosity, non binary failure reporting, 
> removal of
> - *     crazy_fans thread, added game_length argument by Darren Hart.
> - *      2007-08-01 Remove all thread cleanup in favor of simply 
> exiting.Various
> - *      bugfixes and cleanups. -- Josh Triplett
> + *                crazy_fans thread, added game_length argument by 
> Darren Hart.
> + *     2007-08-01 Remove all thread cleanup in favor of simply 
> exiting.Various
> + *                bugfixes and cleanups. -- Josh Triplett
> + *     2009-06-23 Simplified atomic startup mechanism, avoiding 
> thundering herd
> + *                scheduling at the beginning of the game. -- Darren Hart
>  *
>  *****************************************************************************/ 
> 
> 
> @@ -71,11 +73,9 @@
> /* Here's the position of the ball */
> volatile int the_ball;
> 
> -/* pthread_barrier for synchronization */
> -pthread_barrier_t barrier;
> -
> static int players_per_team = 0;
> static int game_length = DEF_GAME_LENGTH;
> +static atomic_t players_ready;
> 
> void usage(void)
> {
> @@ -110,8 +110,7 @@
> /* This is the defensive team. They're trying to block the offense */
> void *thread_defense(void* arg)
> {
> -    pthread_barrier_wait(&barrier);
> -
> +    atomic_inc(&players_ready);
>     /*keep the ball from being moved */
>     while (1) {
>         sched_yield(); /* let other defenders run */
> @@ -123,8 +122,7 @@
> /* This is the offensive team. They're trying to move the ball */
> void *thread_offense(void* arg)
> {
> -    pthread_barrier_wait(&barrier);
> -
> +    atomic_inc(&players_ready);
>     while (1) {
>         the_ball++; /* move the ball ahead one yard */
>         sched_yield(); /* let other offensive players run */
> @@ -141,9 +139,9 @@
> 
>     gettimeofday(&start, NULL);
>     now = start;
> -    the_ball = 0;
> 
> -    pthread_barrier_wait(&barrier);
> +    /* Start the game! */
> +    the_ball = 0;
> 
>     /* Watch the game */
>     while ((now.tv_sec - start.tv_sec) < game_length) {
> @@ -169,10 +167,7 @@
>     if (players_per_team == 0)
>         players_per_team = sysconf(_SC_NPROCESSORS_ONLN);
> 
> -    if ((i = pthread_barrier_init(&barrier, NULL, players_per_team*2 + 
> 1))) {
> -        printf("pthread_barrier_init failed: %s\n", strerror(i));
> -        exit(i);
> -    }
> +    atomic_set(0, &players_ready);
> 
>     printf("Running with: players_per_team=%d game_length=%d\n",
>            players_per_team, game_length);
> @@ -181,13 +176,20 @@
>     param.sched_priority = sched_get_priority_min(SCHED_FIFO) + 80;
>     sched_setscheduler(0, SCHED_FIFO, &param);
> 
> -    /* Start the offense */
> +    /*
> +     * Start the offense
> +     * They are lower priority than defense, so they must be started 
> first.
> +     */
>     priority = 15;
>     printf("Starting %d offense threads at priority %d\n",
>             players_per_team, priority);
>     for (i = 0; i < players_per_team; i++)
>         create_fifo_thread(thread_offense, NULL, priority);
> 
> +    /* Wait for the offense threads to start */
> +    while (atomic_get(&players_ready) < players_per_team)
> +        usleep(100);
> +
>     /* Start the defense */
>     priority = 30;
>     printf("Starting %d defense threads at priority %d\n",
> @@ -195,6 +197,10 @@
>     for (i = 0; i < players_per_team; i++)
>         create_fifo_thread(thread_defense, NULL, priority);
> 
> +    /* Wait for the defense threads to start */
> +    while (atomic_get(&players_ready) < players_per_team * 2)
> +        usleep(100);
> +
>     /* Ok, everyone is on the field, bring out the ref */
>     printf("Starting referee thread\n");
>     return referee(game_length);


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