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 14:47:10 +0200 [thread overview]
Message-ID: <f2e9187a-dea8-ef55-b815-9ac295b46919@suse.cz> (raw)
In-Reply-To: <20210525123536.GR30378@techsingularity.net>
On 5/25/21 2:35 PM, Mel Gorman wrote:
> On Tue, May 25, 2021 at 01:39:29AM +0200, Vlastimil Babka wrote:
>> Currently __slab_alloc() disables irqs around the whole ___slab_alloc(). This
>> includes cases where this is not needed, such as when the allocation ends up in
>> the page allocator and has to awkwardly enable irqs back based on gfp flags.
>> Also the whole kmem_cache_alloc_bulk() is executed with irqs disabled even when
>> it hits the __slab_alloc() slow path, and long periods with disabled interrupts
>> are undesirable.
>>
>> As a first step towards reducing irq disabled periods, move irq handling into
>> ___slab_alloc(). Callers will instead prevent the s->cpu_slab percpu pointer
>> from becoming invalid via migrate_disable(). This does not protect against
>> access preemption, which is still done by disabled irq for most of
>> ___slab_alloc(). As the small immediate benefit, slab_out_of_memory() call from
>> ___slab_alloc() is now done with irqs enabled.
>>
>> kmem_cache_alloc_bulk() disables irqs for its fastpath and then re-enables them
>> before calling ___slab_alloc(), which then disables them at its discretion. The
>> whole kmem_cache_alloc_bulk() operation also disables cpu migration.
>>
>> When ___slab_alloc() calls new_slab() to allocate a new page, re-enable
>> preemption, because new_slab() will re-enable interrupts in contexts that allow
>> blocking.
>>
>> The patch itself will thus increase overhead a bit due to disabled migration
>> and increased disabling/enabling irqs in kmem_cache_alloc_bulk(), but that will
>> be gradually improved in the following patches.
>>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>
> 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.
> 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?
> On that basis, I'd recommend starting with preempt_disable
> and only using migrate_disable if necessary.
That's certainly possible and you're right it would be a less disruptive step.
My thinking was that on !PREEMPT_RT it's actually just preempt_disable (however
with the call overhead currently), but PREEMPT_RT would welcome the lack of
preempt disable. I'd be interested to hear RT guys opinion here.
> Bonus points for adding a comment where ___slab_alloc disables IRQs to
> clarify what is protected -- I assume it's protecting kmem_cache_cpu
> from being modified from interrupt context. If so, it's potentially a
> local_lock candidate.
Yeah that gets cleared up later :)
next prev parent reply other threads:[~2021-05-25 12:47 UTC|newest]
Thread overview: 53+ 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 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 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 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 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
2021-06-08 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 [this message]
2021-05-25 15:10 ` Mel Gorman
2021-05-25 17:24 ` Vlastimil Babka
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
2021-05-24 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
2021-06-09 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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