Re: [patch 12/19] softirq: Add RT specific softirq accounting

Frederic Weisbecker
2020-11-19T22:52:52Z

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!