• Thomas Gleixner's avatar
    hrtimer: Initialize CLOCK_ID to HRTIMER_BASE table statically · ce31332d
    Thomas Gleixner authored
    Sedat and Bruno reported RCU stalls which turned out to be caused by
    the following;
    
    sched_init() calls init_rt_bandwidth() which calls hrtimer_init()
    _BEFORE_ hrtimers_init() is called. While not entirely correct this
    worked because hrtimer_init() only accessed statically initialized
    data (hrtimer_bases.clock_base[CLOCK_MONOTONIC])
    
    Commit e06383db (hrtimers: extend hrtimer base code to handle more
    then 2 clockids) added an indirection to the hrtimer_bases.clock_base
    lookup to avoid gap handling in the hot path. The table which is used
    for the translataion from CLOCK_ID to HRTIMER_BASE index is
    initialized at runtime in hrtimers_init(). So the early call of the
    scheduler code translates CLOCK_MONOTONIC to HRTIMER_BASE_REALTIME.
    
    Thus the rt_bandwith timer ends up on CLOCK_REALTIME. If the timer is
    armed and the wall clock time is set (e.g. ntpdate in the early boot
    process - which also gives the problem deterministic behaviour
    i.e. magic recovery after N hours), then the timer ends up with an
    expiry time far into the future. That breaks the RT throttler
    mechanism as rt runtime is accumulated and never cleared, so the rt
    throttler detects a false cpu hog condition and blocks all RT tasks
    until the timer finally expires. That in turn stalls the RCU thread of
    TINYRCU which leads to an huge amount of RCU callbacks piling up.
    
    Make the translation table statically initialized, so we are back to
    the status of <= 2.6.39.
    Reported-and-tested-by: default avatarSedat Dilek <sedat.dilek@gmail.com>
    Reported-by: default avatarBruno Prémont <bonbons@linux-vserver.org>
    Cc: John stultz <johnstul@us.ibm.com>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/%3Calpine.LFD.2.02.1104282353140.3005%40ionos%3EReviewed-by: default avatarIngo Molnar <mingo@elte.hu>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    ce31332d
hrtimer.c 46.1 KB