Re: [RFC PATCH 0/7] locking/rtqspinlock: Realtime queued spinlocks

[Daniel Bristot de Oliveira <bristot@redhat.com>]

On 01/05/2017 05:08 PM, Peter Zijlstra wrote:
> RT very fundamentally relies on the spinlock->rtmutex conversion to
> allow preempting things when a higher priority task comes along. A
> spinlock, of any kind, requires having preemption disabled while holding
> the lock. If the critical section is of unbounded latency, you have
> unbounded preemption latency and RT is no more.
> 
> Its not about PI on contention, although that helps inversion scenarios.
> Its about allowing preemption, which fundamentally requires a sleeping
> lock to be used.

2 cents complementing this:

Spinlocks algorithms which disable the preemption while holding the lock
resembles a theoretical algorithm named Immediate Priority Ceiling
Protocol. The immediate priority ceiling protocol avoids unbounded
priority inversion, like the Priority Inheritance Protocol used on rt
mutex. However, although the Immediate Priority Ceiling Protocol may
help to reduce the response time of tasks, it causes penalty on the
activation/scheduling delay (latency on linux -rt dictionary) of tasks
with the priority higher than the task holding the lock, but lower than
the Ceiling priority. As it is not possible to know on beforehand the
Ceiling priority of a lock on Linux, the implementation needs to use the
highest priority, that is only possible by disabling the preemption on
Linux, causing scheduling latency even for the highest -rt task.

Hence, the penalty of the immediate priority ceiling protocol is right
in the main metric of the PREEMPT_RT: the scheduling latency.

This is an old article:

http://www.jsoftware.us/vol7/jsw0703-03.pdf

for uni processors... so it does not completely fit in this case but it
shows some results of using immediate priority ceiling rather than PI on
rt-mutex. It shows that the immediate priority ceiling causes scheduling
delays/latency

This paper mentions the Multicore Priority Ceiling Protocol causing
scheduling latency as well:

https://people.mpi-sws.org/~bbb/papers/pdf/rtlws12.pdf

That is why RT Mutex with PIP is better for the PREEMPT than any
protocol that disables preemption.

-- Daniel