Messages in this thread | | | Date | Mon, 4 Mar 2019 16:48:16 +0000 | From | Quentin Perret <> | Subject | Re: 答复: [PATCH V4] sc hed/cpufreq: initializ e iowait_boost_max and iowait_boost with cpu capacity |
| |
On Monday 04 Mar 2019 at 16:26:16 (+0100), Peter Zijlstra wrote: > On Mon, Mar 04, 2019 at 01:58:12PM +0000, Quentin Perret wrote: > > You could also update the values in sugov_get_util() at the cost of a > > small overhead to compute 'min'. I'm not sure what's preferable since > > we wanted to avoid that kind of overhead in the first place ... > > Or,... we could actually make things simpler. > > How's the below? I have a feq questions wrt min, mostly: > > - what's the difference between policy->min and > policy->cpuinfo.min_freq; it used to be the former, the below uses > the latter.
As mentioned on IRC, IIRC policy->min is something that can be written from userspace (for example) to cap the min freq. OTOH, cpuinfo.min_freq is read-only and just reports the lowest OPP.
Rafael is this correct ?
> - should we have a min_freq based value, instead of a constant; the > difference being that with this the actual boost speed depends in the > gap between min/max.
If the above is correct, then I agree. Looking at min_freq simplifies things quite a bit since it doesn't need to be updated all the time, and the whole policy->min stuff is dealt with at the CPUFreq core level so it's not obvious sugov should care.
> Anyway; the below converts iowait_boost to capacity scale (from kHz), it > side-steps the whole issue you guys are bickering about by limiting it > to SCHED_CAPACITY_SCALE (aka. 1024) on the boost path, and then limiting > it to @max by the time we figured out we ought to use it. > > And since that means we never change @max anymore; we can simplify whole > return value thing.
[...]
> @@ -837,7 +818,9 @@ static int sugov_start(struct cpufreq_policy *policy) > memset(sg_cpu, 0, sizeof(*sg_cpu)); > sg_cpu->cpu = cpu; > sg_cpu->sg_policy = sg_policy; > - sg_cpu->iowait_boost_max = policy->cpuinfo.max_freq; > + sg_cpu->min = > + (SCHED_CAPACITY_SCALE * policy->cpuinfo.min_freq) / > + policy->cpuinfo.max_freq;
The 'funny' thing is that on big little this 'min' can end up being larger than 'max' ...
On juno r0 for example, min_freq and max_freq for little CPUs are respectively 450MHz and 850MHz. So you get sg_cpu->min=542, but sg_cpu->max=446 ... So you'll max out after the first IO wakeup. And since iowait_boost is reset whenever it is smaller than sg_cpu->min, you end up with something that can either force max freq or apply no boost at all ...
Perhaps you could keep the 'util' and 'max' pointers in sugov_iowait_apply() and overwrite them like before, but in the SCHED_CAPACITY_SCALE scale as you suggest ?
Thanks, Quentin
| |