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

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Date:   Thu, 18 Aug 2022 16:37:06 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christoph Lameter <cl@...two.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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 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.

> fastpath does not require disabling preemption or interrupts if the
> processor supports local rmv operations. So you should be able to use the
> fastpath on PREEMPT_RT.
> 
> If the fastpath is not possible then you need to disable preemption and
> eventually take locks etc and then things may get a bit more complicated.

Yeah that's basically the solution for PREEMPT_RT, take the local_lock.