From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx> [ Upstream commit 2317d5f1c34913bac5971d93d69fb6c31bb74670 ] I was testing Daniel's changes with his test case, and tweaked it a little. Instead of having the runtime equal to the deadline, I increased the deadline ten fold. Daniel's test case had: attr.sched_runtime = 2 * 1000 * 1000; /* 2 ms */ attr.sched_deadline = 2 * 1000 * 1000; /* 2 ms */ attr.sched_period = 2 * 1000 * 1000 * 1000; /* 2 s */ To make it more interesting, I changed it to: attr.sched_runtime = 2 * 1000 * 1000; /* 2 ms */ attr.sched_deadline = 20 * 1000 * 1000; /* 20 ms */ attr.sched_period = 2 * 1000 * 1000 * 1000; /* 2 s */ The results were rather surprising. The behavior that Daniel's patch was fixing came back. The task started using much more than .1% of the CPU. More like 20%. Looking into this I found that it was due to the dl_entity_overflow() constantly returning true. That's because it uses the relative period against relative runtime vs the absolute deadline against absolute runtime. runtime / (deadline - t) > dl_runtime / dl_period There's even a comment mentioning this, and saying that when relative deadline equals relative period, that the equation is the same as using deadline instead of period. That comment is backwards! What we really want is: runtime / (deadline - t) > dl_runtime / dl_deadline We care about if the runtime can make its deadline, not its period. And then we can say "when the deadline equals the period, the equation is the same as using dl_period instead of dl_deadline". After correcting this, now when the task gets enqueued, it can throttle correctly, and Daniel's fix to the throttling of sleeping deadline tasks works even when the runtime and deadline are not the same. Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Reviewed-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> Cc: Juri Lelli <juri.lelli@xxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx> Cc: Mike Galbraith <efault@xxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Romulo Silva de Oliveira <romulo.deoliveira@xxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Tommaso Cucinotta <tommaso.cucinotta@xxxxxxxx> Link: http://lkml.kernel.org/r/02135a27f1ae3fe5fd032568a5a2f370e190e8d7.1488392936.git.bristot@xxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxx> --- kernel/sched/deadline.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 6d594e22bbc8..df5c32a0c6ed 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -445,13 +445,13 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se, * * This function returns true if: * - * runtime / (deadline - t) > dl_runtime / dl_period , + * runtime / (deadline - t) > dl_runtime / dl_deadline , * * IOW we can't recycle current parameters. * - * Notice that the bandwidth check is done against the period. For + * Notice that the bandwidth check is done against the deadline. For * task with deadline equal to period this is the same of using - * dl_deadline instead of dl_period in the equation above. + * dl_period instead of dl_deadline in the equation above. */ static bool dl_entity_overflow(struct sched_dl_entity *dl_se, struct sched_dl_entity *pi_se, u64 t) @@ -476,7 +476,7 @@ static bool dl_entity_overflow(struct sched_dl_entity *dl_se, * of anything below microseconds resolution is actually fiction * (but still we want to give the user that illusion >;). */ - left = (pi_se->dl_period >> DL_SCALE) * (dl_se->runtime >> DL_SCALE); + left = (pi_se->dl_deadline >> DL_SCALE) * (dl_se->runtime >> DL_SCALE); right = ((dl_se->deadline - t) >> DL_SCALE) * (pi_se->dl_runtime >> DL_SCALE); -- 2.11.0