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    Subject[RFC PATCH] Turn off the tick even when not idle
    The following patch (not for application any time soon) hacks away the
    timer interrupt even when not idle, by triggering the nohz mechanism
    even if not running the idle task.

    When a process does some number crunching for a while, without involving
    the kernel, the kernel still interrupts it HZ times per second to figure
    out if it has any work to do. With a system dedicated to doing such
    number crunching, the answer will almost always come up "no"; however,
    the kernel takes a while figuring out all the "no"s from various
    subsystems, every timer tick. On my system, the timer tick takes about
    80us, every 1/HZ seconds; that represents a significant overhead. 80us
    out of every 1ms, for instance, means 8% overhead. Furthermore, the
    time taken varies, and the timer interrupts lead to jitter in the
    performance of the number crunching.

    This patch represents an attempt to demonstrate the effect of removing
    the timer interrupt. It by no means represents a complete solution; it
    just thwacks the timer interrupt over the head, ignoring the various
    things it does. Known issues include breaking RCU, process accounting
    (using "300%" of one CPU), and POSIX CPU timers, among other things. I
    have some fixes in progress for some of those.

    Nevertheless, this patch successfully boots, runs, and demonstrates some
    good results. I ran the benchmark "Fixed Time Quantum" (ftq), which
    repeatedly runs fixed length intervals and counts how many iterations of
    a simple loop it can run within those intervals. I've attached a plot
    of the results with HZ=1000, HZ=250, and this nohz hack; also available
    at along with the raw numbers.
    I sorted the samples by iterations completed, to group similar values
    together. (The ~5 bad samples on the far left represent unavoidable
    SMIs on the laptop I ran the tests on.)

    Notice how with the timer tick turned off, the results show long
    "shelves" of near-identical values. More than half the samples fall
    into one such shelf, consistently completing almost the same hundreds of
    thousands of iterations within ~20 iterations of each other. With the
    timer tick turned on, the results spread out a lot more, in the
    direction of worse performance.

    Please, give this patch a try and let me know what you think.

    I'd like to work towards a patch which really can kill off the timer
    tick, making the kernel entirely event-driven and removing the polling
    that occurs in the timer tick. I've reviewed everything the timer tick
    does, and every last bit of it could occur using an event-driven

    - Josh Triplett

    -- >8 --

    kernel/softirq.c | 2 +-
    kernel/time/tick-sched.c | 8 --------
    2 files changed, 1 insertions(+), 9 deletions(-)

    diff --git a/kernel/softirq.c b/kernel/softirq.c
    index eb5e131..8bf11b4 100644
    --- a/kernel/softirq.c
    +++ b/kernel/softirq.c
    @@ -305,7 +305,7 @@ void irq_exit(void)
    #ifdef CONFIG_NO_HZ
    /* Make sure that timer wheel updates are propagated */
    - if (idle_cpu(smp_processor_id()) && !in_interrupt() && !need_resched())
    + if (!in_interrupt() && !need_resched())
    diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    index e0f59a2..707ba98 100644
    --- a/kernel/time/tick-sched.c
    +++ b/kernel/time/tick-sched.c
    @@ -223,14 +223,6 @@ void tick_nohz_stop_sched_tick(int inidle)
    cpu = smp_processor_id();
    ts = &per_cpu(tick_cpu_sched, cpu);

    - /*
    - * Call to tick_nohz_start_idle stops the last_update_time from being
    - * updated. Thus, it must not be called in the event we are called from
    - * irq_exit() with the prior state different than idle.
    - */
    - if (!inidle && !ts->inidle)
    - goto end;
    now = tick_nohz_start_idle(ts);

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