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sched/deadline: Improve admission control for asymmetric CPU capacities

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The current SCHED_DEADLINE (DL) admission control ensures that

    sum of reserved CPU bandwidth < x * M

where

    x = /proc/sys/kernel/sched_rt_{runtime,period}_us
    M = # CPUs in root domain.

DL admission control works well for homogeneous systems where the
capacity of all CPUs are equal (1024). I.e. bounded tardiness for DL
and non-starvation of non-DL tasks is guaranteed.

But on heterogeneous systems where capacity of CPUs are different it
could fail by over-allocating CPU time on smaller capacity CPUs.

On an Arm big.LITTLE/DynamIQ system DL tasks can easily starve other
tasks making it unusable.

Fix this by explicitly considering the CPU capacity in the DL admission
test by replacing M with the root domain CPU capacity sum.

Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200520134243.19352-4-dietmar.eggemann@arm.com
This commit is contained in:
Luca Abeni
2020-05-20 15:42:41 +02:00
committed by Peter Zijlstra
parent fc9dc69847
commit 60ffd5edc5
2 changed files with 20 additions and 16 deletions
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6
kernel/sched/sched.h
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@@ -310,11 +310,11 @@ void __dl_add(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
__dl_update(dl_b, -((s32)tsk_bw / cpus));
}
static inline
bool __dl_overflow(struct dl_bw *dl_b, int cpus, u64 old_bw, u64 new_bw)
static inline bool __dl_overflow(struct dl_bw *dl_b, unsigned long cap,
u64 old_bw, u64 new_bw)
{
return dl_b->bw != -1 &&
dl_b->bw * cpus < dl_b->total_bw - old_bw + new_bw;
cap_scale(dl_b->bw, cap) < dl_b->total_bw - old_bw + new_bw;
}
extern void init_dl_bw(struct dl_bw *dl_b);