• Ionela Voinescu's avatar
    cpufreq: move invariance setter calls in cpufreq core · 1a0419b0
    Ionela Voinescu authored
    To properly scale its per-entity load-tracking signals, the task scheduler
    needs to be given a frequency scale factor, i.e. some image of the current
    frequency the CPU is running at. Currently, this scale can be computed
    either by using counters (APERF/MPERF on x86, AMU on arm64), or by
    piggy-backing on the frequency selection done by cpufreq.
    
    For the latter, drivers have to explicitly set the scale factor
    themselves, despite it being purely boiler-plate code: the required
    information depends entirely on the kind of frequency switch callback
    implemented by the driver, i.e. either of: target_index(), target(),
    fast_switch() and setpolicy().
    
    The fitness of those callbacks with regard to driving the Frequency
    Invariance Engine (FIE) is studied below:
    
    target_index()
    ==============
    Documentation states that the chosen frequency "must be determined by
    freq_table[index].frequency". It isn't clear if it *has* to be that
    frequency, or if it can use that frequency value to do some computation
    that ultimately leads to a different frequency selection. All drivers
    go for the former, while the vexpress-spc-cpufreq has an atypical
    implementation which is handled separately.
    
    Therefore, the hook works on the assumption the core can use
    freq_table[index].frequency.
    
    target()
    =======
    This has been flagged as deprecated since:
    
      commit 9c0ebcf7 ("cpufreq: Implement light weight ->target_index() routine")
    
    It also doesn't have that many users:
    
      gx-suspmod.c:439:       .target = cpufreq_gx_target,
      s3c24xx-cpufreq.c:428:  .target = s3c_cpufreq_target,
      intel_pstate.c:2528:    .target = intel_cpufreq_target,
      cppc_cpufreq.c:401:     .target = cppc_cpufreq_set_target,
      cpufreq-nforce2.c:371:  .target = nforce2_target,
      sh-cpufreq.c:163:       .target = sh_cpufreq_target,
      pcc-cpufreq.c:573:      .target = pcc_cpufreq_target,
    
    Similarly to the path taken for target_index() calls in the cpufreq core
    during a frequency change, all of the drivers above will mark the end of a
    frequency change by a call to cpufreq_freq_transition_end().
    
    Therefore, cpufreq_freq_transition_end() can be used as the location for
    the arch_set_freq_scale() call to potentially inform the scheduler of the
    frequency change.
    
    This change maintains the previous functionality for the drivers that
    implement the target_index() callback, while also adding support for the
    few drivers that implement the deprecated target() callback.
    
    fast_switch()
    =============
    This callback *has* to return the frequency that was selected.
    
    setpolicy()
    ===========
    This callback does not have any designated way of informing what was the
    end choice. But there are only two drivers using setpolicy(), and none
    of them have current FIE support:
    
      drivers/cpufreq/longrun.c:281:	.setpolicy	= longrun_set_policy,
      drivers/cpufreq/intel_pstate.c:2215:	.setpolicy	= intel_pstate_set_policy,
    
    The intel_pstate is known to use counter-driven frequency invariance.
    
    Conclusion
    ==========
    
    Given that the significant majority of current FIE enabled drivers use
    callbacks that lend themselves to triggering the setting of the FIE scale
    factor in a generic way, move the invariance setter calls to cpufreq core.
    
    As a result of setting the frequency scale factor in cpufreq core, after
    callbacks that lend themselves to trigger it, remove this functionality
    from the driver side.
    
    To be noted that despite marking a successful frequency change, many
    cpufreq drivers will consider the new frequency as the requested
    frequency, although this is might not be the one granted by the hardware.
    
    Therefore, the call to arch_set_freq_scale() is a "best effort" one, and
    it is up to the architecture if the new frequency is used in the new
    frequency scale factor setting (determined by the implementation of
    arch_set_freq_scale()) or eventually used by the scheduler (determined
    by the implementation of arch_scale_freq_capacity()). The architecture
    is in a better position to decide if it has better methods to obtain
    more accurate information regarding the current frequency and use that
    information instead (for example, the use of counters).
    
    Also, the implementation to arch_set_freq_scale() will now have to handle
    error conditions (current frequency == 0) in order to prevent the
    overhead in cpufreq core when the default arch_set_freq_scale()
    implementation is used.
    Signed-off-by: default avatarIonela Voinescu <ionela.voinescu@arm.com>
    Suggested-by: default avatarValentin Schneider <valentin.schneider@arm.com>
    Acked-by: default avatarViresh Kumar <viresh.kumar@linaro.org>
    Acked-by: default avatarSudeep Holla <sudeep.holla@arm.com>
    Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
    1a0419b0
cpufreq-dt.c 9.47 KB