Messages in this thread | | | Subject | Re: [PATCH] sched: rt: Make RT capacity aware | From | Dietmar Eggemann <> | Date | Mon, 7 Oct 2019 11:14:11 +0200 |
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On 23/09/2019 13:52, Qais Yousef wrote: > On 09/20/19 14:52, Dietmar Eggemann wrote: >>> 2. The fallback mechanism means we either have to call cpupri_find() >>> twice once to find filtered lowest_rq and the other to return the >>> none filtered version. >> >> This is what I have in mind. (Only compile tested! ... and the 'if >> (cpumask_any(lowest_mask) >= nr_cpu_ids)' condition has to be considered >> as well): >> >> @@ -98,8 +103,26 @@ int cpupri_find(struct cpupri *cp, struct >> task_struct *p, >> continue; >> >> if (lowest_mask) { >> + int cpu, max_cap_cpu = -1; >> + unsigned long max_cap = 0; >> + >> cpumask_and(lowest_mask, p->cpus_ptr, vec->mask); >> >> + for_each_cpu(cpu, lowest_mask) { >> + unsigned long cap = >> arch_scale_cpu_capacity(cpu); >> + >> + if (!rt_task_fits_capacity(p, cpu)) >> + cpumask_clear_cpu(cpu, lowest_mask); >> + >> + if (cap > max_cap) { >> + max_cap = cap; >> + max_cap_cpu = cpu; >> + } >> + } >> + >> + if (cpumask_empty(lowest_mask) && max_cap) >> + cpumask_set_cpu(max_cap_cpu, lowest_mask); > > I had a patch that I was testing but what I did is to continue rather than > return a max_cap_cpu.
Continuing is the correct thing to do here. I just tried to illustrate the idea.
> e.g: > > if no cpu at current priority fits the task: > continue; > else: > return the lowest_mask which contains fitting cpus only > > if no fitting cpu was find: > return 0;
I guess this is what we want to achieve here. It's unavoidable that we will run sooner (compared to an SMP system) into a situation in which we have to go higher in the rd->cpupri->pri_to_cpu[] array or in which we can't return a lower mask at all.
> Or we can tweak your approach to be > > if no cpu at current priority fits the task: > if the cpu the task is currently running on doesn't fit it: > return lowest_mask with max_cap_cpu set;
I wasn't aware of the pri_to_cpu[] array and how cpupri_find(, lowest_mask) tries to return the lowest_mask of the lowest priority (pri_to_cpu[] index).
> So we either: > > 1. Continue the search until we find a fitting CPU; bail out otherwise.
If this describes the solution in which we concentrate the capacity-awareness in cpupri_find(), then I'm OK with it. find_lowest_rq() already favours task_cpu(task), this_cpu and finally cpus in sched_groups (from the viewpoint of task_cpu(task)).
> 2. Or we attempt to return a CPU only if the CPU the task is currently > running on doesn't fit it. We don't want to migrate the task from a > fitting to a non-fitting one.
I would prefer 1., keeping the necessary changes confined in cpupri_find() if possible.
> We can also do something hybrid like: > > 3. Remember the outcome of 2 but don't return immediately and attempt > to find a fitting CPU at a different priority level. > > > Personally I see 1 is the simplest and good enough solution. What do you think?
Agreed. We would potentially need a fast lookup for p -> uclamp_cpumask though?
> I think this is 'continue' to search makes doing it at cpupri_find() more > robust than having to deal with whatever mask we first found in > find_lowest_rq() - so I'm starting to like this approach better. Thanks for > bringing it up.
My main concern is that having rt_task_fits_capacity() added to almost every condition in the code makes it hard to understand what capacity awareness in RT wants to achieve.
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