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

Joel Fernandes
2023-03-03T19:26:43Z

On Fri, Mar 3, 2023 at 1:37 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri, 3 Mar 2023 18:11:34 +0000
> Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > In the normal mutex's adaptive spinning, there is no check for if there is a
> > change in waiter AFAICS (ignoring ww mutex stuff for a second).
> >
> > I can see one may want to do that waiter-check, as spinning
> > indefinitely if the lock owner is on the CPU for too long may result in
> > excessing power burn. But normal mutex does not seem to do that.
> >
> > What makes the rtmutex spin logic different from normal mutex in this
> > scenario, so that rtmutex wants to do that but normal ones dont?
>
> Well, the point of the patch is that I don't think they should be different
> ;-)

But there's no "waiter change" thing for mutex_spin_on_owner right.

Then, should mutex_spin_on_owner() also add a call to
__mutex_waiter_is_first() ?

> > Another thought is, I am wondering if all of them spinning indefinitely might
> > be Ok for rtmutex as well, since as you mentioned, preemption is enabled. So
> > adding the if (top_waiter != last_waiter) {...} might be unnecessary? In fact
> > may be even harmful as you are disabling interrupts in the process.
>
> The last patch only does the interrupt disabling if the top waiter changes.
> Which in practice is seldom.
>
> But, I don't think a non top waiter should spin if the top waiter is not
> running. The top waiter is the one that will get the lock next. If the
> owner releases the lock and gives it to the top waiter, then it has to go
> through the wake up of the top waiter.

Correct me if I'm wrong, but I don't think it will go through
schedule() after spinning, which is what adds the overhead for John.

> I don't see why a task should spin
> to save a wake up if a wake up has to happen anyway.

What about regular mutexes, happens there too or no?

> > Either way, I think a comment should go on top of the "if (top_waiter !=
> > waiter)" check IMO.
>
> What type of comment?

Comment explaining why "if (top_waiter != waiter)" is essential :-).

thanks,

-Joel