On Mon, 4 Mar 2019 at 17:48, Quentin Perret <[email protected]> wrote: > > 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' ...
yes arch_scale_cpu_capacity() should be used instead of SCHED_CAPACITY_SCALE to be compared with sg_cpu->max and sg_cpu->util > > 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
