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

[Subrata Modak <subrata@linux.vnet.ibm.com>]

On Thu, 2009-06-25 at 13:34 -0700, Darren Hart wrote: 
> Subrata Modak wrote:
> > On Thu, 2009-06-25 at 09:46 +0530, gowrishankar wrote: 
> >> 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>
> >>>
> > 
> > Thanks Darren. Long time LTP say updates to RT tests. What happened to
> > the RT-Graphics tests that probably were in the pipeline ?
> 
> Hrm.... well.... I can't recall what those might have been.  The only 
> graphics testing we do is really just functional testing using existing 
> tests like x11perf and specviewperf.  We haven't written anything 
> specifically for -rt.  Do you have any more detail on what this might 
> have been?

Not sure, cannot remember. But, i might have seen it in one of our
internal mails/status/plan. Never mind. 

Regards--
Subrata

> 
> --
> Darren
> 
> > 
> > Regards--
> > Subrata
> > 
> >> 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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