• Linus Torvalds's avatar
    Merge branch 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip · 534c97b0
    Linus Torvalds authored
    Pull 'full dynticks' support from Ingo Molnar:
     "This tree from Frederic Weisbecker adds a new, (exciting! :-) core
      kernel feature to the timer and scheduler subsystems: 'full dynticks',
      or CONFIG_NO_HZ_FULL=y.
    
      This feature extends the nohz variable-size timer tick feature from
      idle to busy CPUs (running at most one task) as well, potentially
      reducing the number of timer interrupts significantly.
    
      This feature got motivated by real-time folks and the -rt tree, but
      the general utility and motivation of full-dynticks runs wider than
      that:
    
       - HPC workloads get faster: CPUs running a single task should be able
         to utilize a maximum amount of CPU power.  A periodic timer tick at
         HZ=1000 can cause a constant overhead of up to 1.0%.  This feature
         removes that overhead - and speeds up the system by 0.5%-1.0% on
         typical distro configs even on modern systems.
    
       - Real-time workload latency reduction: CPUs running critical tasks
         should experience as little jitter as possible.  The last remaining
         source of kernel-related jitter was the periodic timer tick.
    
       - A single task executing on a CPU is a pretty common situation,
         especially with an increasing number of cores/CPUs, so this feature
         helps desktop and mobile workloads as well.
    
      The cost of the feature is mainly related to increased timer
      reprogramming overhead when a CPU switches its tick period, and thus
      slightly longer to-idle and from-idle latency.
    
      Configuration-wise a third mode of operation is added to the existing
      two NOHZ kconfig modes:
    
       - CONFIG_HZ_PERIODIC: [formerly !CONFIG_NO_HZ], now explicitly named
         as a config option.  This is the traditional Linux periodic tick
         design: there's a HZ tick going on all the time, regardless of
         whether a CPU is idle or not.
    
       - CONFIG_NO_HZ_IDLE: [formerly CONFIG_NO_HZ=y], this turns off the
         periodic tick when a CPU enters idle mode.
    
       - CONFIG_NO_HZ_FULL: this new mode, in addition to turning off the
         tick when a CPU is idle, also slows the tick down to 1 Hz (one
         timer interrupt per second) when only a single task is running on a
         CPU.
    
      The .config behavior is compatible: existing !CONFIG_NO_HZ and
      CONFIG_NO_HZ=y settings get translated to the new values, without the
      user having to configure anything.  CONFIG_NO_HZ_FULL is turned off by
      default.
    
      This feature is based on a lot of infrastructure work that has been
      steadily going upstream in the last 2-3 cycles: related RCU support
      and non-periodic cputime support in particular is upstream already.
    
      This tree adds the final pieces and activates the feature.  The pull
      request is marked RFC because:
    
       - it's marked 64-bit only at the moment - the 32-bit support patch is
         small but did not get ready in time.
    
       - it has a number of fresh commits that came in after the merge
         window.  The overwhelming majority of commits are from before the
         merge window, but still some aspects of the tree are fresh and so I
         marked it RFC.
    
       - it's a pretty wide-reaching feature with lots of effects - and
         while the components have been in testing for some time, the full
         combination is still not very widely used.  That it's default-off
         should reduce its regression abilities and obviously there are no
         known regressions with CONFIG_NO_HZ_FULL=y enabled either.
    
       - the feature is not completely idempotent: there is no 100%
         equivalent replacement for a periodic scheduler/timer tick.  In
         particular there's ongoing work to map out and reduce its effects
         on scheduler load-balancing and statistics.  This should not impact
         correctness though, there are no known regressions related to this
         feature at this point.
    
       - it's a pretty ambitious feature that with time will likely be
         enabled by most Linux distros, and we'd like you to make input on
         its design/implementation, if you dislike some aspect we missed.
         Without flaming us to crisp! :-)
    
      Future plans:
    
       - there's ongoing work to reduce 1Hz to 0Hz, to essentially shut off
         the periodic tick altogether when there's a single busy task on a
         CPU.  We'd first like 1 Hz to be exposed more widely before we go
         for the 0 Hz target though.
    
       - once we reach 0 Hz we can remove the periodic tick assumption from
         nr_running>=2 as well, by essentially interrupting busy tasks only
         as frequently as the sched_latency constraints require us to do -
         once every 4-40 msecs, depending on nr_running.
    
      I am personally leaning towards biting the bullet and doing this in
      v3.10, like the -rt tree this effort has been going on for too long -
      but the final word is up to you as usual.
    
      More technical details can be found in Documentation/timers/NO_HZ.txt"
    
    * 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (39 commits)
      sched: Keep at least 1 tick per second for active dynticks tasks
      rcu: Fix full dynticks' dependency on wide RCU nocb mode
      nohz: Protect smp_processor_id() in tick_nohz_task_switch()
      nohz_full: Add documentation.
      cputime_nsecs: use math64.h for nsec resolution conversion helpers
      nohz: Select VIRT_CPU_ACCOUNTING_GEN from full dynticks config
      nohz: Reduce overhead under high-freq idling patterns
      nohz: Remove full dynticks' superfluous dependency on RCU tree
      nohz: Fix unavailable tick_stop tracepoint in dynticks idle
      nohz: Add basic tracing
      nohz: Select wide RCU nocb for full dynticks
      nohz: Disable the tick when irq resume in full dynticks CPU
      nohz: Re-evaluate the tick for the new task after a context switch
      nohz: Prepare to stop the tick on irq exit
      nohz: Implement full dynticks kick
      nohz: Re-evaluate the tick from the scheduler IPI
      sched: New helper to prevent from stopping the tick in full dynticks
      sched: Kick full dynticks CPU that have more than one task enqueued.
      perf: New helper to prevent full dynticks CPUs from stopping tick
      perf: Kick full dynticks CPU if events rotation is needed
      ...
    534c97b0
governors.txt 11 KB