Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted

Hyeonggon Yoo
2022-08-25T05:15:20Z

On Fri, Aug 19, 2022 at 05:04:31PM +0200, Christoph Lameter wrote:
> On Thu, 18 Aug 2022, Vlastimil Babka wrote:
>
> > On 8/18/22 11:42, Christoph Lameter wrote:
> > > On Wed, 17 Aug 2022, Sebastian Andrzej Siewior wrote:
> > >
> > >> + * On PREEMPT_RT, the local lock neither disables interrupts nor preemption
> > >> + * which means the lockless fastpath cannot be used as it might interfere with
> > >> + * an in-progress slow path operations. In this case the local lock is always
> > >> + * taken but it still utilizes the freelist for the common operations.
> > >
> > > The slub fastpath does not interfere with slow path operations and the
> >
> > That's true on !PREEMPT_RT because a slowpath operation under
> > local_lock_irqsave() will disable interrupts, so there can't be a
> > fastpath operation in an interrupt handler appearing in the middle of a
> > slowpath operation.
> >
> > On PREEMPT_RT local_lock_irqsave() doesn't actually disable interrupts,
> > so that can happen. IIRC we learned that the hard way when Mike
> > Galbraith was testing early versions of my PREEMPT_RT changes for SLUB.
>
> Well yes if you enable interrupts during the slowpath then interrupts may
> use the fastpath. That is a basic design change to the way concurrency is
> handled in the allocators.
>
> There needs to be some fix here to restore the exclusion of the fastpath
> during slow path processing. This could be
>
> A) Exclude the fastpath during slowpath operations
>
> This can be accomplished by setting things up like in the debug mode
> that also excludes the fastpath.

I think we can do that by disabling preemption (for a short period, I think)
in slowpath on RT (like disabling irq in non-RT)

But I wonder if RT guys will prefer that?

> B) Force interrupt allocations to the slowpath.
>
> Check some flag that indicates an interrupt allocation is occurring and
> then bypass the fastpath.

There is nothing special about interrupt allocation on RT.
All users of SLUB on RT must not be in hardirq context.

So I don't think it is possible to distingush between a thread being preempted
and another thread that preempts it.

--
Thanks,
Hyeonggon