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Date:   Fri, 19 Aug 2022 17:04:31 +0200 (CEST)
From:   Christoph Lameter <cl@gentwo.de>
To:     Vlastimil Babka <vbabka@suse.cz>
cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Pekka Enberg <penberg@kernel.org>, linux-mm@kvack.org
Subject: Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted
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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.

or

B) Force interrupt allocations to the slowpath.

Check some flag that indicates an interrupt allocation is occurring and
then bypass the fastpath.