Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for CONFIG_PARAVIRT

[Waiman Long]

On 1/7/25 8:59 AM, Kumar Kartikeya Dwivedi wrote:
> We ripped out PV and virtualization related bits from rqspinlock in an
> earlier commit, however, a fair lock performs poorly within a virtual
> machine when the lock holder is preempted. As such, retain the
> virt_spin_lock fallback to test and set lock, but with timeout and
> deadlock detection.
>
> We don't integrate support for CONFIG_PARAVIRT_SPINLOCKS yet, as that
> requires more involved algorithmic changes and introduces more
> complexity. It can be done when the need arises in the future.

virt_spin_lock() doesn't scale well. It is for hypervisors that don't 
support PV qspinlock yet. Now rqspinlock() will be in this category.

I wonder if we should provide an option to disable rqspinlock and fall 
back to the regular qspinlock with strict BPF locking semantics.

Another question that I have is about PREEMPT_RT kernel which cannot 
tolerate any locking stall. That will probably require disabling 
rqspinlock if CONFIG_PREEMPT_RT is enabled.

Cheers,
Longman