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    idle governor: Avoid lock acquisition to read pm_qos before entering idle · 333c5ae9
    Tim Chen 提交于
    Thanks to the reviews and comments by Rafael, James, Mark and Andi.
    Here's version 2 of the patch incorporating your comments and also some
    update to my previous patch comments.
    
    I noticed that before entering idle state, the menu idle governor will
    look up the current pm_qos target value according to the list of qos
    requests received.  This look up currently needs the acquisition of a
    lock to access the list of qos requests to find the qos target value,
    slowing down the entrance into idle state due to contention by multiple
    cpus to access this list.  The contention is severe when there are a lot
    of cpus waking and going into idle.  For example, for a simple workload
    that has 32 pair of processes ping ponging messages to each other, where
    64 cpu cores are active in test system, I see the following profile with
    37.82% of cpu cycles spent in contention of pm_qos_lock:
    
    -     37.82%          swapper  [kernel.kallsyms]          [k]
    _raw_spin_lock_irqsave
       - _raw_spin_lock_irqsave
          - 95.65% pm_qos_request
               menu_select
               cpuidle_idle_call
             - cpu_idle
                  99.98% start_secondary
    
    A better approach will be to cache the updated pm_qos target value so
    reading it does not require lock acquisition as in the patch below.
    With this patch the contention for pm_qos_lock is removed and I saw a
    2.2X increase in throughput for my message passing workload.
    
    cc: stable@kernel.org
    Signed-off-by: NTim Chen <tim.c.chen@linux.intel.com>
    Acked-by: NAndi Kleen <ak@linux.intel.com>
    Acked-by: NJames Bottomley <James.Bottomley@suse.de>
    Acked-by: Nmark gross <markgross@thegnar.org>
    Signed-off-by: NLen Brown <len.brown@intel.com>
    333c5ae9
pm_qos_params.h 1.2 KB