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    SubjectRe: [patch 12/19] softirq: Add RT specific softirq accounting
    On Thu, Nov 19, 2020 at 07:34:13PM +0100, Thomas Gleixner wrote:
    > On Thu, Nov 19 2020 at 13:18, Frederic Weisbecker wrote:
    > > On Fri, Nov 13, 2020 at 03:02:19PM +0100, Thomas Gleixner wrote:
    > >> RT requires the softirq to be preemptible and uses a per CPU local lock to
    > >> protect BH disabled sections and softirq processing. Therefore RT cannot
    > >> use the preempt counter to keep track of BH disabled/serving.
    > >>
    > >> Add a RT only counter to task struct and adjust the relevant macros in
    > >> preempt.h.
    > >
    > > You may want to describe a bit the reason for this per task counter.
    > > It's not intuitive at this stage.
    >
    > Something like this:
    >
    > RT requires the softirq processing and local bottomhalf disabled regions
    > to be preemptible. Using the normal preempt count based serialization is
    > therefore not possible because this implicitely disables preemption.
    >
    > RT kernels use a per CPU local lock to serialize bottomhalfs. As
    > local_bh_disable() can nest the lock can only be acquired on the
    > outermost invocation of local_bh_disable() and released when the nest
    > count becomes zero. Tasks which hold the local lock can be preempted so
    > its required to keep track of the nest count per task.
    >
    > Add a RT only counter to task struct and adjust the relevant macros in
    > preempt.h.
    >
    > Thanks,

    Very good, thanks!

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