1. 13 Nov, 2009 2 commits
  2. 05 Nov, 2009 2 commits
    • Martin Schwidefsky's avatar
      nohz: Introduce arch_needs_cpu · 3c5d92a0
      Martin Schwidefsky authored
      Allow the architecture to request a normal jiffy tick when the system
      goes idle and tick_nohz_stop_sched_tick is called . On s390 the hook is
      used to prevent the system going fully idle if there has been an
      interrupt other than a clock comparator interrupt since the last wakeup.
      
      On s390 the HiperSockets response time for 1 connection ping-pong goes
      down from 42 to 34 microseconds. The CPU cost decreases by 27%.
      Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
      LKML-Reference: <20090929122533.402715150@de.ibm.com>
      Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
      3c5d92a0
    • Martin Schwidefsky's avatar
      nohz: Reuse ktime in sub-functions of tick_check_idle. · eed3b9cf
      Martin Schwidefsky authored
      On a system with NOHZ=y tick_check_idle calls tick_nohz_stop_idle and
      tick_nohz_update_jiffies. Given the right conditions (ts->idle_active
      and/or ts->tick_stopped) both function get a time stamp with ktime_get.
      The same time stamp can be reused if both function require one.
      
      On s390 this change has the additional benefit that gcc inlines the
      tick_nohz_stop_idle function into tick_check_idle. The number of
      instructions to execute tick_check_idle drops from 225 to 144
      (without the ktime_get optimization it is 367 vs 215 instructions).
      
      before:
      
       0)               |  tick_check_idle() {
       0)               |    tick_nohz_stop_idle() {
       0)               |      ktime_get() {
       0)               |        read_tod_clock() {
       0)   0.601 us    |        }
       0)   1.765 us    |      }
       0)   3.047 us    |    }
       0)               |    ktime_get() {
       0)               |      read_tod_clock() {
       0)   0.570 us    |      }
       0)   1.727 us    |    }
       0)               |    tick_do_update_jiffies64() {
       0)   0.609 us    |    }
       0)   8.055 us    |  }
      
      after:
      
       0)               |  tick_check_idle() {
       0)               |    ktime_get() {
       0)               |      read_tod_clock() {
       0)   0.617 us    |      }
       0)   1.773 us    |    }
       0)               |    tick_do_update_jiffies64() {
       0)   0.593 us    |    }
       0)   4.477 us    |  }
      Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
      Cc: john stultz <johnstul@us.ibm.com>
      LKML-Reference: <20090929122533.206589318@de.ibm.com>
      Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
      eed3b9cf
  3. 05 Oct, 2009 2 commits
    • john stultz's avatar
      time: Remove xtime_cache · 7bc7d637
      john stultz authored
      With the prior logarithmic time accumulation patch, xtime will now
      always be within one "tick" of the current time, instead of
      possibly half a second off.
      
      This removes the need for the xtime_cache value, which always
      stored the time at the last interrupt, so this patch cleans that up
      removing the xtime_cache related code.
      
      This is a bit simpler, but still could use some wider testing.
      Signed-off-by: default avatarJohn Stultz <johnstul@us.ibm.com>
      Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
      Reviewed-by: default avatarJohn Kacur <jkacur@redhat.com>
      Cc: Clark Williams <williams@redhat.com>
      Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
      Cc: Andrew Morton <akpm@linux-foundation.org>
      LKML-Reference: <1254525855.7741.95.camel@localhost.localdomain>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      7bc7d637
    • john stultz's avatar
      time: Implement logarithmic time accumulation · a092ff0f
      john stultz authored
      Accumulating one tick at a time works well unless we're using NOHZ.
      Then it can be an issue, since we may have to run through the loop
      a few thousand times, which can increase timer interrupt caused
      latency.
      
      The current solution was to accumulate in half-second intervals
      with NOHZ. This kept the number of loops down, however it did
      slightly change how we make NTP adjustments. While not an issue
      with NTPd users, as NTPd makes adjustments over a longer period of
      time, other adjtimex() users have noticed the half-second
      granularity with which we can apply frequency changes to the clock.
      
      For instance, if a application tries to apply a 100ppm frequency
      correction for 20ms to correct a 2us offset, with NOHZ they either
      get no correction, or a 50us correction.
      
      Now, there will always be some granularity error for applying
      frequency corrections. However with users sensitive to this error
      have seen a 50-500x increase with NOHZ compared to running without
      NOHZ.
      
      So I figured I'd try another approach then just simply increasing
      the interval. My approach is to consume the time interval
      logarithmically. This reduces the number of times through the loop
      needed keeping latency down, while still preserving the original
      granularity error for adjtimex() changes.
      
      Further, this change allows us to remove the xtime_cache code
      (patch to follow), as xtime is always within one tick of the
      current time, instead of the half-second updates it saw before.
      
      An earlier version of this patch has been shipping to x86 users in
      the RedHat MRG releases for awhile without issue, but I've reworked
      this version to be even more careful about avoiding possible
      overflows if the shift value gets too large.
      Signed-off-by: default avatarJohn Stultz <johnstul@us.ibm.com>
      Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
      Reviewed-by: default avatarJohn Kacur <jkacur@redhat.com>
      Cc: Clark Williams <williams@redhat.com>
      Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
      Cc: Andrew Morton <akpm@linux-foundation.org>
      LKML-Reference: <1254525473.7741.88.camel@localhost.localdomain>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      a092ff0f
  4. 04 Oct, 2009 8 commits
  5. 03 Oct, 2009 6 commits
  6. 02 Oct, 2009 20 commits