Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq

[Mathieu Desnoyers]

On 9/28/23 07:22, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 28 September 2023 11:39
> >
> > On Mon, May 29, 2023 at 03:14:13PM -0400, Mathieu Desnoyers wrote:
> > > Expose the "on-cpu" state for each thread through struct rseq to allow
> > > adaptative mutexes to decide more accurately between busy-waiting and
> > > calling sys_futex() to release the CPU, based on the on-cpu state of the
> > > mutex owner.
>
> Are you trying to avoid spinning when the owning process is sleeping?

Yes, this is my main intent.

> Or trying to avoid the system call when it will find that the futex
> is no longer held?
>
> The latter is really horribly detremental.

That's a good questions. What should we do in those three situations 
when trying to grab the lock:

1) Lock has no owner

We probably want to simply grab the lock with an atomic instruction. But 
then if other threads are queued on sys_futex and did not manage to grab 
the lock yet, this would be detrimental to fairness.

2) Lock owner is running:

The lock owner is certainly running on another cpu (I'm using the term 
"cpu" here as logical cpu).

I guess we could either decide to bypass sys_futex entirely and try to 
grab the lock with an atomic, or we go through sys_futex nevertheless to 
allow futex to guarantee some fairness across threads.

3) Lock owner is sleeping:

The lock owner may be either tied to the same cpu as the requester, or a 
different cpu. Here calling FUTEX_WAIT and friends is pretty much required.

Can you elaborate on why skipping sys_futex in scenario (2) would be so 
bad ? I wonder if we could get away with skipping futex entirely in this 
scenario and still guarantee fairness by implementing MCS locking or 
ticket locks in userspace. Basically, if userspace queues itself on the 
lock through either MCS locking or ticket locks, it could guarantee 
fairness on its own.

Of course things are more complicated with PI-futex, is that what you 
have in mind ?

> > > It is only provided as an optimization hint, because there is no
> > > guarantee that the page containing this field is in the page cache, and
> > > therefore the scheduler may very well fail to clear the on-cpu state on
> > > preemption. This is expected to be rare though, and is resolved as soon
> > > as the task returns to user-space.
> > >
> > > The goal is to improve use-cases where the duration of the critical
> > > sections for a given lock follows a multi-modal distribution, preventing
> > > statistical guesses from doing a good job at choosing between busy-wait
> > > and futex wait behavior.
> >
> > As always, are syscalls really *that* expensive? Why can't we busy wait
> > in the kernel instead?
> >
> > I mean, sure, meltdown sucked, but most people should now be running
> > chips that are not affected by that particular horror show, no?
>
> IIRC 'page table separation' which is what makes system calls expensive
> is only a compile-time option. So is likely to be enabled on any 'distro'
> kernel.
> But a lot of other mitigations (eg RSB stuffing) are also pretty detrimental.
>
> OTOH if you have a 'hot' userspace mutex you are going to lose whatever.
> All that needs to happen is for a ethernet interrupt to decide to discard
> completed transmits and refill the rx ring, and then for the softint code
> to free a load of stuff deferred by rcu while you've grabbed the mutex
> and no matter how short the user-space code path the mutex won't be
> released for absolutely ages.
>
> I had to change a load of code to use arrays and atomic increments
> to avoid delays acquiring mutex.

That's good input, thanks! I mostly defer to André Almeida on the 
use-case motivation. I mostly provided this POC patch to show that it 
_can_ be done with sys_rseq(2).

Thanks!

Mathieu

> 	David
>
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Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com