Re: [PATCH] pstore: Revert pmsg_lock back to a normal mutex

Joel Fernandes
2023-03-07T20:19:42Z

On Tue, Mar 7, 2023 at 9:09 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Mar 03, 2023 at 06:11:34PM +0000, Joel Fernandes wrote:
> > What makes the rtmutex spin logic different from normal mutex in this
> > scenario, so that rtmutex wants to do that but normal ones dont?
>
> Regular mutex uses osq 'lock' to serialize waiters and only the top
> spinner gets to spin on the mutex itself, this greatly reduces the
> contention on the mutex.
>
> OSQ is FIFO, which is not what RT-mutex needs.

Got it, so basically OSQ ensures fairness as its FIFO and also reduces
lock contention because I am guessing the OSQ-spinners are spinning on
a per-spinner MCS node (that per-CPU optimistic_spin_node it appears).

This makes perfect sense now, thank you Peter!!!

- Joel