From: zihan zhou 15645113830zzh@gmail.com
[ Upstream commit 2ae891b826958b60919ea21c727f77bcd6ffcc2c ]
The old default value for slice is 0.75 msec * (1 + ilog(ncpus)) which means that we have a default slice of:
0.75 for 1 cpu 1.50 up to 3 cpus 2.25 up to 7 cpus 3.00 for 8 cpus and above.
For HZ=250 and HZ=100, because of the tick accuracy, the runtime of tasks is far higher than their slice.
For HZ=1000 with 8 cpus or more, the accuracy of tick is already satisfactory, but there is still an issue that tasks will get an extra tick because the tick often arrives a little faster than expected. In this case, the task can only wait until the next tick to consider that it has reached its deadline, and will run 1ms longer.
vruntime + sysctl_sched_base_slice = deadline |-----------|-----------|-----------|-----------| 1ms 1ms 1ms 1ms ^ ^ ^ ^ tick1 tick2 tick3 tick4(nearly 4ms)
There are two reasons for tick error: clockevent precision and the CONFIG_IRQ_TIME_ACCOUNTING/CONFIG_PARAVIRT_TIME_ACCOUNTING. with CONFIG_IRQ_TIME_ACCOUNTING every tick will be less than 1ms, but even without it, because of clockevent precision, tick still often less than 1ms.
In order to make scheduling more precise, we changed 0.75 to 0.70, Using 0.70 instead of 0.75 should not change much for other configs and would fix this issue:
0.70 for 1 cpu 1.40 up to 3 cpus 2.10 up to 7 cpus 2.8 for 8 cpus and above.
This does not guarantee that tasks can run the slice time accurately every time, but occasionally running an extra tick has little impact.
Signed-off-by: zihan zhou 15645113830zzh@gmail.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Vincent Guittot vincent.guittot@linaro.org Link: https://lkml.kernel.org/r/20250208075322.13139-1-15645113830zzh@gmail.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ceb023629d48d..7a2c94e8f0b2c 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -71,10 +71,10 @@ unsigned int sysctl_sched_tunable_scaling = SCHED_TUNABLESCALING_LOG; /* * Minimal preemption granularity for CPU-bound tasks: * - * (default: 0.75 msec * (1 + ilog(ncpus)), units: nanoseconds) + * (default: 0.70 msec * (1 + ilog(ncpus)), units: nanoseconds) */ -unsigned int sysctl_sched_base_slice = 750000ULL; -static unsigned int normalized_sysctl_sched_base_slice = 750000ULL; +unsigned int sysctl_sched_base_slice = 700000ULL; +static unsigned int normalized_sysctl_sched_base_slice = 700000ULL;
const_debug unsigned int sysctl_sched_migration_cost = 500000UL;