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

Joel Fernandes
2023-03-04T03:01:37Z

Hey Steve,

On Fri, Mar 03, 2023 at 02:38:22PM -0500, Steven Rostedt wrote:
> On Fri, 3 Mar 2023 14:25:23 -0500
> Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > 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() ?
>
> Ah interesting, I missed the __mutex_waiter_is_first() in the mutex code,
> where it looks to do basically the same thing as rt_mutex (but slightly
> different).

True, I concur!

> From looking at this, it appears that mutex() has FIFO fair
> logic, where the second waiter will sleep.
>
> Would be interesting to see why John sees such a huge difference between
> normal mutex and rtmutex if they are doing the same thing. One thing is
> perhaps the priority logic is causing the issue, where this will not
> improve anything.

> I wonder if we add spinning to normal mutex for the other waiters if that
> would improve things or make them worse?

Yeah it could improve things (or make them worse ;-). In that approach, I
guess those other waiters will not be spinning for too long once the first
waiter gets the lock, as mutex_spin_on_owner() it will break out of the spin:

while (__mutex_owner(lock) == owner) {
...
}

But yeah it sounds like a plausible idea.

> > > > 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.

Ah ok. I see what you're doing now!

I guess that check will help whenever the top-waiter keeps changing. In that
case you want only the latest top-waiter as the spinner, all while the lock
owner is not changing.

> > 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.
>
> Only if the lock becomes free.

Ah yeah, true!

> > > 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?
>
> Yes, but in a FIFO order, where in rt_mutex, a second, higher priority task
> can make the first ones sleep. So maybe it's just the priority logic that
> is causing the issues.

True! So in the FIFO thing, there's no way a high prio task can get ahead in
line of the first waiter, makes sense.

> > > > 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 :-).
>

Maybe "/* Only the top waiter needs to spin. If we are no longer the
top-waiter, no point in spinning, as we do not get the lock next anyway. */"

?

thanks,

- Joel