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Date:   Thu, 18 Aug 2022 11:42:00 +0200 (CEST)
From:   Christoph Lameter <cl@gentwo.de>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
cc:     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>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org
Subject: Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted
In-Reply-To: <20220817162703.728679-2-bigeasy@linutronix.de>
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References: <20220817162703.728679-1-bigeasy@linutronix.de> <20220817162703.728679-2-bigeasy@linutronix.de>
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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
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.