linux-kernel - Re: [PATCH 1/9] slub: Make PREEMPT

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Date:   Thu, 25 Aug 2022 14:15:06 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Christoph Lameter <cl@...two.de>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted

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