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From: Vlastimil Babka <vbabka@suse.cz>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Christoph Lameter <cl@linux.com>,
	David Rientjes <rientjes@google.com>,
	Pekka Enberg <penberg@kernel.org>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jann Horn <jannh@google.com>
Subject: Re: [RFC 09/26] mm, slub: move disabling/enabling irqs to ___slab_alloc()
Date: Tue, 25 May 2021 19:24:44 +0200	[thread overview]
Message-ID: <1c9027d9-c6d7-f05d-49a4-a6396a59280c@suse.cz> (raw)
In-Reply-To: <f2e9187a-dea8-ef55-b815-9ac295b46919@suse.cz>

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.


  parent reply	other threads:[~2021-05-25 17:24 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-24 23:39 [RFC 00/26] SLUB: use local_lock for kmem_cache_cpu protection and reduce disabling irqs Vlastimil Babka
2021-05-24 23:39 ` [RFC 01/26] mm, slub: allocate private object map for sysfs listings Vlastimil Babka
2021-05-25  8:06   ` Christoph Lameter
2021-05-25  8:06     ` Christoph Lameter
2021-05-25 10:13   ` Mel Gorman
2021-05-24 23:39 ` [RFC 02/26] mm, slub: allocate private object map for validate_slab_cache() Vlastimil Babka
2021-05-25  8:09   ` Christoph Lameter
2021-05-25  8:09     ` Christoph Lameter
2021-05-25 10:17   ` Mel Gorman
2021-05-25 10:36     ` Vlastimil Babka
2021-05-25 11:33       ` Mel Gorman
2021-06-08 10:37         ` Vlastimil Babka
2021-05-24 23:39 ` [RFC 03/26] mm, slub: don't disable irq for debug_check_no_locks_freed() Vlastimil Babka
2021-05-25 10:24   ` Mel Gorman
2021-05-24 23:39 ` [RFC 04/26] mm, slub: simplify kmem_cache_cpu and tid setup Vlastimil Babka
2021-05-25 11:47   ` Mel Gorman
2021-05-24 23:39 ` [RFC 05/26] mm, slub: extract get_partial() from new_slab_objects() Vlastimil Babka
2021-05-25  9:03   ` Christoph Lameter
2021-05-25  9:03     ` Christoph Lameter
2021-05-25 11:54   ` Mel Gorman
2021-05-24 23:39 ` [RFC 06/26] mm, slub: dissolve new_slab_objects() into ___slab_alloc() Vlastimil Babka
2021-05-25  9:06   ` Christoph Lameter
2021-05-25  9:06     ` Christoph Lameter
2021-05-25 11:59   ` Mel Gorman
2021-05-24 23:39 ` [RFC 07/26] mm, slub: return slab page from get_partial() and set c->page afterwards Vlastimil Babka
2021-05-25  9:12   ` Christoph Lameter
2021-05-25  9:12     ` Christoph Lameter
2021-06-08 10:48     ` Vlastimil Babka
2021-05-24 23:39 ` [RFC 08/26] mm, slub: restructure new page checks in ___slab_alloc() Vlastimil Babka
2021-05-25 12:09   ` Mel Gorman
2021-05-24 23:39 ` [RFC 09/26] mm, slub: move disabling/enabling irqs to ___slab_alloc() Vlastimil Babka
2021-05-25 12:35   ` Mel Gorman
2021-05-25 12:47     ` Vlastimil Babka
2021-05-25 15:10       ` Mel Gorman
2021-05-25 17:24       ` Vlastimil Babka [this message]
2021-05-24 23:39 ` [RFC 10/26] mm, slub: do initial checks in ___slab_alloc() with irqs enabled Vlastimil Babka
2021-05-25 13:04   ` Mel Gorman
2021-06-08 12:13     ` Vlastimil Babka
2021-05-24 23:39 ` [RFC 11/26] mm, slub: move disabling irqs closer to get_partial() in ___slab_alloc() Vlastimil Babka
2021-05-25 16:00   ` Jann Horn
2021-05-25 16:00     ` Jann Horn
2021-05-24 23:39 ` [RFC 12/26] mm, slub: restore irqs around calling new_slab() Vlastimil Babka
2021-05-24 23:39 ` [RFC 13/26] mm, slub: validate partial and newly allocated slabs before loading them Vlastimil Babka
2021-05-24 23:39 ` [RFC 14/26] mm, slub: check new pages with restored irqs Vlastimil Babka
2021-05-24 23:39 ` [RFC 15/26] mm, slub: stop disabling irqs around get_partial() Vlastimil Babka
2021-05-24 23:39 ` [RFC 16/26] mm, slub: move reset of c->page and freelist out of deactivate_slab() Vlastimil Babka
2021-05-24 23:39 ` [RFC 17/26] mm, slub: make locking in deactivate_slab() irq-safe Vlastimil Babka
2021-05-24 23:39 ` [RFC 18/26] mm, slub: call deactivate_slab() without disabling irqs Vlastimil Babka
2021-05-24 23:39 ` [RFC 19/26] mm, slub: move irq control into unfreeze_partials() Vlastimil Babka
2021-05-24 23:39 ` [RFC 20/26] mm, slub: discard slabs in unfreeze_partials() without irqs disabled Vlastimil Babka
2021-05-24 23:39 ` [RFC 21/26] mm, slub: detach whole partial list at once in unfreeze_partials() Vlastimil Babka
2021-05-24 23:39 ` [RFC 22/26] mm, slub: detach percpu partial list in unfreeze_partials() using this_cpu_cmpxchg() Vlastimil Babka
2021-05-24 23:39 ` [RFC 23/26] mm, slub: only disable irq with spin_lock in __unfreeze_partials() Vlastimil Babka
2021-05-24 23:39 ` [RFC 24/26] mm, slub: don't disable irqs in slub_cpu_dead() Vlastimil Babka
2021-05-24 23:39 ` [RFC 25/26] mm, slub: use migrate_disable() in put_cpu_partial() Vlastimil Babka
2021-05-25 15:33   ` Jann Horn
2021-05-25 15:33     ` Jann Horn
2021-06-09  8:41     ` Vlastimil Babka
2021-05-24 23:39 ` [RFC 26/26] mm, slub: convert kmem_cpu_slab protection to local_lock Vlastimil Babka
2021-05-25 16:11   ` Vlastimil Babka

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