Messages in this thread | | | Date | Tue, 20 Sep 2022 20:17:29 +0200 | Subject | Re: [PATCH v4 6/8] sched/fair: Add sched group latency support | From | Dietmar Eggemann <> |
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On 19/09/2022 17:49, Vincent Guittot wrote: > On Mon, 19 Sept 2022 at 13:55, Dietmar Eggemann > <dietmar.eggemann@arm.com> wrote: >> >> s/valentin.schneider@arm.com// >> >> On 16/09/2022 10:03, Vincent Guittot wrote: >>> Task can set its latency priority, which is then used to decide to preempt >>> the current running entity of the cfs, but sched group entities still have >>> the default latency offset. >>> >>> Add a latency field in task group to set the latency offset of the >>> sched_eneities of the group, which will be used against other entities in >> >> s/sched_eneities/sched_entity >> >>> the parent cfs when deciding which entity to schedule first. >> >> So latency for cgroups does not follow any (existing) Resource >> Distribution Model/Scheme (Documentation/admin-guide/cgroup-v2.rst)? >> Latency values are only used to compare sched entities at the same level. > > Just like share/cpu.weight value does for time sharing
But for this we define it as following the `Weights` scheme. That's why I was asking,
>> [...] >> >>> +static int cpu_latency_write_s64(struct cgroup_subsys_state *css, >>> + struct cftype *cft, s64 latency) >>> +{ >> >> There is no [MIN, MAX] checking? > > This is done is sched_group_set_latency() which checks that > abs(latency) < sysctl_sched_latency
I see. Nit-picking: Wouldn't this allow to specify a latency offset value for the non-existent `nice = 20`? Highest nice value 19 maps to `973/1024 * sysctl_sched_latency`.
> >> >> min_weight = sched_latency_to_weight[0] = -1024 >> max_weight = sched_latency_to_weight[39] = 973 >> >> [MIN, MAX] = [sysctl_sched_latency * min_weight >> NICE_LATENCY_SHIFT, >> sysctl_sched_latency * max_weight >> NICE_LATENCY_SHIFT] >> >> >> With the `cpu.latency` knob user would have to know for example that the >> value is -24,000,000ns to get the same behaviour as for a task latency >> nice = -20 (latency prio = 0) (w/ sysctl_sched_latency = 24ms)? > > Yes, Tejun raised some concerns about adding an interface like nice in > the task group in v2 so I have removed it. > >> >> For `nice` we have `cpu.weight.nice` next to `cpu.weight` in cgroup v2 ? > > If everybody is ok, I can add back the cpu.latency.nice interface in > the v5 in addition to the cpu.latency
cpu.weight/cpu.weight.nice interface:
echo X > cpu.weight tg->shares
1 10,240 100 1,048,576 10000 104,857,600
echo X > cpu.weight.nice
-20 90,891,264 0 1,048,576 19 15,360
Wouldn't then a similar interface for cpu.latency [1..100..10000] and cpu.latency.nice [-20..0..19] make most sense?
Raw latency_offset values at interface level are not portable.
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