Re: [PATCH V4 0/3] sched/deadline: Fixes for constrained deadline tasks

[Daniel Bristot de Oliveira]

Hi,

This is a gentle ping.

On 03/02/2017 03:10 PM, Daniel Bristot de Oliveira wrote:
> While reading sched deadline code, I find out that a constrained
> deadline task could be replenished before the next period if
> activated after the deadline, opening the window to run for more
> than Q/P. The patch [2] explains and fixes this problem.
> 
> Furthermore, while fixing this issue, I found that the replenishment
> timer was being fired at the deadline of the task. This works fine
> for implicit deadline tasks (deadline == period) because the deadline
> is at the same point in time of the next period. But that is not true
> for constrained deadline tasks (deadline < period). This problem is
> not as visible as the first because the runtime leakage takes
> place only in the second activation. Next activations receive the
> correct bandwidth. However, after the 2nd activation, tasks are
> activated in the (period - dl_deadline) instant, which is before
> the expected activation. This problem is explained in the fix
> description as well.
> 
> While testing these fixes, Rostedt tweaked the test case a little.
> Instead of having the runtime equal to the deadline, he increased
> the deadline ten fold. Then, the task started using much more than
> .1% of the CPU. More like 20%. Looking into this he 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. As we care about if the
> runtime can make its deadline, not its period, we need to use the
> task's density in the check, not the task's utilization. After
> correcting this, now when the task gets enqueued, it can throttle
> correctly.
> 
> Changes from V3:
>  - Fixes grammar errors in the patch 2/3. (Steven Rostedt)
>  - I was checking if the pi_se was constrained, not the task being
>    awakened.
>    This was not causing problems in the test case because
>    pi_se = &p->dl, but this would be a problem if we were activating
>    the task in a PI case:
>      It would check the pi-waiter, not the task being awakened (p).
> Changes from V2:
>  - Fixes dl_entity_overflow(): (Steven Rostedt)
>    Patch 3/3 fixes the dl_entity_overflow() for constrained deadline
>    tasks by using the density, not the utilization.
>    (as deadline <= period, deadline is always == min(deadline, period))
> Changes from V1:
>  - Fix a broken comment style. (Peter Zijlstra)
>  - Fixes dl_is_constrained(). (Steven Rostedt)
>     A constrained deadline task has dl_deadline < dl_period; so
> 	"dl_runtime < dl_period"; s/runtime/deadline/
> 
> Daniel Bristot de Oliveira (2):
>   sched/deadline: Replenishment timer should fire in the next period
>   sched/deadline: Throttle a constrained deadline task activated after
>     the deadline
> 
> Steven Rostedt (VMware) (1):
>   sched/deadline: Use deadline instead of period when calculating
>     overflow
> 
>  kernel/sched/deadline.c | 62 ++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 56 insertions(+), 6 deletions(-)
>