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    SubjectRe: [linux-pm] Too many timer interrupts in NO_HZ
    On Sun, 16 Mar 2008, Vaidyanathan Srinivasan wrote:

    > > The problem:
    > >
    > > There are way too many timer interrupts even though the CPUs have
    > > entered tickless idle loop. Timer interrupts basically bring the CPU
    > > out of idle, and then return to tickless idle. There are very few
    > > try_to_wake_up()s or need_resched() in between the timer interrupts.
    > >
    > > What can happen in an idle system in the timer interrupt context that
    > > does not invoke a need_resched() or try_to_wake_up()?

    > Function Count Name
    > Address
    >
    > c0219922 : 5 blk_unplug_timeout
    > c014f464 : 55 wb_timer_fn
    > c02b2e67 : 350 bnx2_timer
    > c03efcbc : 115 neigh_periodic_timer
    > c012894a : 220 process_timeout
    > c027d1c4 : 2 hangcheck_fire
    > c03fe232 : 3 peer_check_expire
    > c012e830 : 25 delayed_work_timer_fn
    > c03afe92 : 2783 ehci_watchdog
    > c03f1a35 : 10 neigh_timer_handler
    > c01d7456 : 110 commit_timeout
    > c04381e9 : 2 addrconf_verify
    > c03f6d39 : 365 dev_watchdog
    > c04126bc : 114 tcp_write_timer

    > These are device driver timers that we need to investigate. We should
    > try to migrate them to CPU0 (or some other package) to get really long
    > uninterrupted CPU sleep time.

    The largest entry is for ehci_watchdog. This timer won't run at all if
    your EHCI controllers are allowed to autosuspend, which will happen
    automatically if

    (1) You enable CONFIG_USB_SUSPEND, and

    (2) You have no high-speed USB devices attached, or the
    ones that are attached have all been suspended.

    On the other hand, if you were actively using some high-speed USB
    device during the test then it's understandable that there should be
    lots of timer interrupts as a result.

    Alan Stern



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