• Shreyas B. Prabhu's avatar
    cpuidle: Fix last_residency division · dbd1b8ea
    Shreyas B. Prabhu authored
    Snooze is a poll idle state in powernv and pseries platforms. Snooze
    has a timeout so that if a CPU stays in snooze for more than target
    residency of the next available idle state, then it would exit
    thereby giving chance to the cpuidle governor to re-evaluate and
    promote the CPU to a deeper idle state. Therefore whenever snooze
    exits due to this timeout, its last_residency will be target_residency
    of the next deeper state.
    
    Commit e93e59ce "cpuidle: Replace ktime_get() with local_clock()"
    changed the math around last_residency calculation. Specifically,
    while converting last_residency value from nano- to microseconds, it
    carries out right shift by 10. Because of that, in snooze timeout
    exit scenarios last_residency calculated is roughly 2.3% less than
    target_residency of the next available state. This pattern is picked
    up by get_typical_interval() in the menu governor and therefore
    expected_interval in menu_select() is frequently less than the
    target_residency of any state other than snooze.
    
    Due to this we are entering snooze at a higher rate, thereby
    affecting the single thread performance.
    
    Fix this by using more precise division via ktime_us_delta().
    
    Fixes: e93e59ce
    
     "cpuidle: Replace ktime_get() with local_clock()"
    Reported-by: default avatarAnton Blanchard <anton@samba.org>
    Bisected-by: default avatarShilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
    Signed-off-by: default avatarShreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
    Acked-by: default avatarDaniel Lezcano <daniel.lezcano@linaro.org>
    Acked-by: default avatarBalbir Singh <bsingharora@gmail.com>
    Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
    dbd1b8ea