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

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

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 ?

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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