Messages in this thread | | | Subject | Re: [PATCH V4 0/3] sched/deadline: Fixes for constrained deadline tasks | From | Daniel Bristot de Oliveira <> | Date | Wed, 15 Mar 2017 17:18:11 +0100 |
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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(-) >
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