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

Subrata Modak
2009-06-26T11:36:07Z

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);
> >