linux-kernel - Re: [RFC 09/26] mm, slub: move disabling/enabling irqs to ___slab

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Date:   Tue, 25 May 2021 19:24:44 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [RFC 09/26] mm, slub: move disabling/enabling irqs to
 ___slab_alloc()

On 5/25/21 2:47 PM, Vlastimil Babka wrote:
> On 5/25/21 2:35 PM, Mel Gorman wrote:
>> 
>> Why did you use migrate_disable instead of preempt_disable? There is a
>> fairly large comment in include/linux/preempt.h on why migrate_disable
>> is undesirable so new users are likely to be put under the microscope
>> once Thomas or Peter notice it.
> 
> I understood it as while undesirable, there's nothing better for now.

Ah I now recalled the more important reason. By my understanding of
Documentation/locking/locktypes.rst it's not possible on PREEMPT_RT to do a
preempt_disable() and then take a spin_lock (or local_lock) which is a mutex on
RT and needs preemption enabled to take it. And one of the goals is that
list_lock would not have to be raw_spinlock on RT anymore.

>> I think you are using it so that an allocation request can be preempted by
>> a higher priority task but given that the code was disabling interrupts,
>> there was already some preemption latency.
> 
> Yes, and the disabled interrupts will get progressively "smaller" in the series.
> 
>> However, migrate_disable
>> is more expensive than preempt_disable (function call versus a simple
>> increment).
> 
> That's true, I think perhaps it could be reimplemented so that on !PREEMPT_RT
> and with no lockdep/preempt/whatnot debugging it could just translate to an
> inline migrate_disable?

Correction: I meant "translate to an inline preempt_disable" which would then
not change anything for !PREEMPT_RT.