Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <5679405a-b3c3-6dc5-783f-7ebeda7c9bf0@suse.cz>
Date:   Thu, 18 Aug 2022 16:37:06 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.11.0
Subject: Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted
Content-Language: en-US
To:     Christoph Lameter <cl@gentwo.de>,
        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>, linux-mm@kvack.org
References: <20220817162703.728679-1-bigeasy@linutronix.de>
 <20220817162703.728679-2-bigeasy@linutronix.de>
 <alpine.DEB.2.22.394.2208181136560.1901102@gentwo.de>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <alpine.DEB.2.22.394.2208181136560.1901102@gentwo.de>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.