• Thomas Gleixner's avatar
    timer: Reduce timer migration overhead if disabled · bc7a34b8
    Thomas Gleixner authored
    Eric reported that the timer_migration sysctl is not really nice
    performance wise as it needs to check at every timer insertion whether
    the feature is enabled or not. Further the check does not live in the
    timer code, so we have an extra function call which checks an extra
    cache line to figure out that it is disabled.
    
    We can do better and store that information in the per cpu (hr)timer
    bases. I pondered to use a static key, but that's a nightmare to
    update from the nohz code and the timer base cache line is hot anyway
    when we select a timer base.
    
    The old logic enabled the timer migration unconditionally if
    CONFIG_NO_HZ was set even if nohz was disabled on the kernel command
    line.
    
    With this modification, we start off with migration disabled. The user
    visible sysctl is still set to enabled. If the kernel switches to NOHZ
    migration is enabled, if the user did not disable it via the sysctl
    prior to the switch. If nohz=off is on the kernel command line,
    migration stays disabled no matter what.
    
    Before:
      47.76%  hog       [.] main
      14.84%  [kernel]  [k] _raw_spin_lock_irqsave
       9.55%  [kernel]  [k] _raw_spin_unlock_irqrestore
       6.71%  [kernel]  [k] mod_timer
       6.24%  [kernel]  [k] lock_timer_base.isra.38
       3.76%  [kernel]  [k] detach_if_pending
       3.71%  [kernel]  [k] del_timer
       2.50%  [kernel]  [k] internal_add_timer
       1.51%  [kernel]  [k] get_nohz_timer_target
       1.28%  [kernel]  [k] __internal_add_timer
       0.78%  [kernel]  [k] timerfn
       0.48%  [kernel]  [k] wake_up_nohz_cpu
    
    After:
      48.10%  hog       [.] main
      15.25%  [kernel]  [k] _raw_spin_lock_irqsave
       9.76%  [kernel]  [k] _raw_spin_unlock_irqrestore
       6.50%  [kernel]  [k] mod_timer
       6.44%  [kernel]  [k] lock_timer_base.isra.38
       3.87%  [kernel]  [k] detach_if_pending
       3.80%  [kernel]  [k] del_timer
       2.67%  [kernel]  [k] internal_add_timer
       1.33%  [kernel]  [k] __internal_add_timer
       0.73%  [kernel]  [k] timerfn
       0.54%  [kernel]  [k] wake_up_nohz_cpu
    Reported-by: default avatarEric Dumazet <edumazet@google.com>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Viresh Kumar <viresh.kumar@linaro.org>
    Cc: John Stultz <john.stultz@linaro.org>
    Cc: Joonwoo Park <joonwoop@codeaurora.org>
    Cc: Wenbo Wang <wenbo.wang@memblaze.com>
    Link: http://lkml.kernel.org/r/20150526224512.127050787@linutronix.deSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    bc7a34b8
core.c 199 KB