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    SubjectRe: [PATCH v2 04/11] sched: Allow all archs to set the power_orig
    On 4 June 2014 11:42, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
    > [...]
    >>> (1) We assume that the current way (update_cpu_power() calls
    >>> arch_scale_freq_power() to get the avg power(freq) over the time period
    >>> since the last call to arch_scale_freq_power()) is suitable
    >>> for us. Do you have another opinion here?
    >>
    >> Using power (or power_freq as you mentioned below) is probably the
    >> easiest and more straight forward solution. You can use it to scale
    >> each element when updating entity runnable.
    >> Nevertheless, I see to 2 potential issues:
    >> - is power updated often enough to correctly follow the frequency
    >> scaling ? we need to compare power update frequency with
    >> runnable_avg_sum variation speed and the rate at which we will change
    >> the CPU's frequency.
    >> - the max value of runnable_avg_sum will be also scaled so a task
    >> running on a CPU with less capacity could be seen as a "low" load even
    >> if it's an always running tasks. So we need to find a way to reach the
    >> max value for such situation
    >
    > I think I mixed two problems together here:
    >
    > Firstly, we need to scale cpu power in update_cpu_power() regarding
    > uArch, frequency and rt/irq pressure.
    > Here the freq related value we get back from arch_scale_freq_power(...,
    > cpu) could be an instantaneous value (curr_freq(cpu)/max_freq(cpu)).
    >
    > Secondly, to be able to scale the runnable avg sum of a sched entity
    > (se->avg->runnable_avg_sum), we preferable have a coefficient
    > representing uArch diffs (cpu_power_orig(cpu)/cpu_power_orig(most
    > powerful cpu in the system) and another coefficient (avg freq over 'now

    AFAICT, the coefficient representing uArch diffs is already taken into
    account into power_freq thanks to scale_cpu, isn't it ?

    > - sa->last_runnable_update'(cpu)/max_freq(cpu). This value would have to
    > be retrieved from the arch in __update_entity_runnable_avg().
    >
    >>> (2) Is the current layout of update_cpu_power() adequate for this, where
    >>> we scale power_orig related to freq and then related to rt/(irq):
    >>>
    >>> power_orig = scale_cpu(SCHED_POWER_SCALE)
    >>> power = scale_rt(scale_freq(power_orig))
    >>>
    >>> or do we need an extra power_freq data member on the rq and do:
    >>>
    >>> power_orig = scale_cpu(SCHED_POWER_SCALE)
    >>> power_freq = scale_freq(power_orig))
    >>> power = scale_rt(power_orig))
    >>
    >> do you really mean power = scale_rt(power_orig) or power=scale_rt(power_freq) ?
    >
    > No, I also think that power=scale_rt(power_freq) is correct.
    >
    >>> In other words, do we consider rt/(irq) pressure when calculating freq
    >>> scale invariant task load or not?
    >>
    >> we should take power_freq which implies a new field
    > [...]
    >


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