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| Started by | Daniel Bristot de Oliveira <bristot@redhat.com> |
|---|---|
| First post | 2017-03-02 15:20 +0100 |
| Last post | 2017-03-07 09:30 +0100 |
| Articles | 2 — 2 participants |
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[PATCH V4 3/3] sched/deadline: Use deadline instead of period when calculating overflow Daniel Bristot de Oliveira <bristot@redhat.com> - 2017-03-02 15:20 +0100
Re: [PATCH V4 3/3] sched/deadline: Use deadline instead of period when calculating overflow Wanpeng Li <kernellwp@gmail.com> - 2017-03-07 09:30 +0100
| From | Daniel Bristot de Oliveira <bristot@redhat.com> |
|---|---|
| Date | 2017-03-02 15:20 +0100 |
| Subject | [PATCH V4 3/3] sched/deadline: Use deadline instead of period when calculating overflow |
| Message-ID | <tgzKh-3oK-5@gated-at.bofh.it> |
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org> 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@goodmis.org> Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Juri Lelli <juri.lelli@arm.com> Cc: Tommaso Cucinotta <tommaso.cucinotta@sssup.it> Cc: Luca Abeni <luca.abeni@santannapisa.it> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Romulo Silva de Oliveira <romulo.deoliveira@ufsc.br> Cc: Daniel Bristot de Oliveira <bristot@redhat.com> Cc: linux-kernel@vger.kernel.org --- 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 b669f7f..f7403e5 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); -- 2.9.3
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| From | Wanpeng Li <kernellwp@gmail.com> |
|---|---|
| Date | 2017-03-07 09:30 +0100 |
| Subject | Re: [PATCH V4 3/3] sched/deadline: Use deadline instead of period when calculating overflow |
| Message-ID | <tiiFk-4KU-9@gated-at.bofh.it> |
| In reply to | #1591123 |
2017-03-02 22:10 GMT+08:00 Daniel Bristot de Oliveira <bristot@redhat.com>: > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org> > > 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@goodmis.org> > Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Juri Lelli <juri.lelli@arm.com> > Cc: Tommaso Cucinotta <tommaso.cucinotta@sssup.it> > Cc: Luca Abeni <luca.abeni@santannapisa.it> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Mike Galbraith <efault@gmx.de> > Cc: Romulo Silva de Oliveira <romulo.deoliveira@ufsc.br> > Cc: Daniel Bristot de Oliveira <bristot@redhat.com> > Cc: linux-kernel@vger.kernel.org > --- Reviewed-by: Wanpeng Li <wanpeng.li@hotmail.com> > 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 b669f7f..f7403e5 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); > > -- > 2.9.3 >
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