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    SubjectRe: [PATCH,RFC] RCU-based detection of stalled CPUs for Classic RCU

    * Paul E. McKenney <> wrote:

    > Hello!
    > This patch adds stalled-CPU detection to Classic RCU. This capability
    > is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR,
    > which defaults disabled. This is a debugging feature, not something
    > that non-kernel-hackers would be expected to care about. This feature
    > can detect looping CPUs in !PREEMPT builds and looping CPUs with
    > preemption disabled in PREEMPT builds. This is essentially a port of
    > this functionality from the treercu patch.
    > One current shortcoming: on some systems, stalls are detected during
    > early boot, when we normally would not care about them. My thought is
    > to add a call from late initialization to suppress stall detection
    > until the system is well along its way to being booted, but thought I
    > should check to see if there might already be something for this
    > purpose.

    could you be a bit more specific, why do those warnings show up and why
    dont we care about them? There are things like networking that
    occasionally do an rcu_sync() and a stall could mean a bootup hang.

    > (Currently against 2.6.27-rc8, FYI.)
    > Thoughts?
    > Signed-off-by: Paul E. McKenney <>

    i think this is a very good idea in general - often the question comes
    up whether a hang seen in the RCU code is indeed caused by RCU or other
    factors. Could you perhaps rebase it against tip/core/rcu ? [or
    tip/master for convenience]


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