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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Vlastimil Babka <vbabka@suse.cz>
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>,
	Thomas Gleixner <tglx@linutronix.de>,
	Mel Gorman <mgorman@techsingularity.net>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jann Horn <jannh@google.com>
Subject: Re: [RFC v2 00/34] SLUB: reduce irq disabled scope and make it RT compatible
Date: Thu, 29 Jul 2021 15:49:39 +0200	[thread overview]
Message-ID: <20210729134939.iulryxjarhjmpugz@linutronix.de> (raw)
In-Reply-To: <35b26e48-a96a-41b0-826e-43e43660c9d6@suse.cz>

now that I'm slowly catching up…

On 2021-07-02 22:25:05 [+0200], Vlastimil Babka wrote:
> > - perf_5.10 stat -r 10 hackbench -g200 -s 4096 -l500
> > Old:
> > |         464.967,20 msec task-clock                #   27,220 CPUs utilized            ( +-  0,16% )
> > New:
> > |         422.865,71 msec task-clock                #    4,782 CPUs utilized            ( +-  0,34% )
> 
> The series shouldn't significantly change the memory allocator
> interaction, though.
> Seems there's less cycles, but more time elapsed, thus more sleeping -
> is it locks becoming mutexes on RT?

yes, most likely since the !RT parts are mostly unchanged.

> My second guess - list_lock remains spinlock with my series, thus RT
> mutex, but the current RT tree converts it to raw_spinlock. I'd hope
> leaving that one as non-raw spinlock would still be much better for RT
> goals, even if hackbench (which is AFAIK very slab intensive) throughput
> regresses - hopefully not that much.

Yes, the list_lock seems to be the case. I picked your
slub-local-lock-v3r0 and changed the list_lock (+slab_lock()) to use
raw_spinlock_t and disable interrupts and CPUs utilisation went to
~23CPUs (plus a bunch of warnings which probably made it a little slower
again).
The difference between a sleeping lock (spinlock_t) and a mutex is
that we attempt not to preempt a task that acquired a spinlock_t even if
it is running for some time and the scheduler would preempt it (like it
would do if the task had a mutex acquired. These are the "lazy preempt"
bits in the RT patch).

By making the list_lock a raw_spinlock_t a lot of IRQ-flags dancing
needs to be done as the page-allocator must be entered with enabled
interrupts. And then there is the possibility that you may need to free
some memory even if you allocate memory which requires some extra steps
on RT due to the IRQ-off part. All this vanishes by keeping list_lock a
spinlock_t.
The kernel-build test on /dev/shm remained unchanged so that is good.
Unless there is a real-world use-case, that gets worse, I don't mind
keeping the spinlock_t here. I haven't seen tglx complaining so far.

Sebastian

  reply	other threads:[~2021-07-29 13:49 UTC|newest]

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-09 11:38 Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 01/34] mm, slub: don't call flush_all() from list_locations() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 02/34] mm, slub: allocate private object map for sysfs listings Vlastimil Babka
2021-06-09 13:29   ` Christoph Lameter
2021-06-09 13:29     ` Christoph Lameter
2021-06-09 11:38 ` [RFC v2 03/34] mm, slub: allocate private object map for validate_slab_cache() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 04/34] mm, slub: don't disable irq for debug_check_no_locks_freed() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 05/34] mm, slub: remove redundant unfreeze_partials() from put_cpu_partial() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 06/34] mm, slub: unify cmpxchg_double_slab() and __cmpxchg_double_slab() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 07/34] mm, slub: extract get_partial() from new_slab_objects() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 08/34] mm, slub: dissolve new_slab_objects() into ___slab_alloc() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 09/34] mm, slub: return slab page from get_partial() and set c->page afterwards Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 10/34] mm, slub: restructure new page checks in ___slab_alloc() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 11/34] mm, slub: simplify kmem_cache_cpu and tid setup Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 12/34] mm, slub: move disabling/enabling irqs to ___slab_alloc() Vlastimil Babka
2021-07-06  4:38   ` Mike Galbraith
2021-07-06  4:38     ` Mike Galbraith
2021-06-09 11:38 ` [RFC v2 13/34] mm, slub: do initial checks in ___slab_alloc() with irqs enabled Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 14/34] mm, slub: move disabling irqs closer to get_partial() in ___slab_alloc() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 15/34] mm, slub: restore irqs around calling new_slab() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 16/34] mm, slub: validate slab from partial list or page allocator before making it cpu slab Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 17/34] mm, slub: check new pages with restored irqs Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 18/34] mm, slub: stop disabling irqs around get_partial() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 19/34] mm, slub: move reset of c->page and freelist out of deactivate_slab() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 20/34] mm, slub: make locking in deactivate_slab() irq-safe Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 21/34] mm, slub: call deactivate_slab() without disabling irqs Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 22/34] mm, slub: move irq control into unfreeze_partials() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 23/34] mm, slub: discard slabs in unfreeze_partials() without irqs disabled Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 24/34] mm, slub: detach whole partial list at once in unfreeze_partials() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 25/34] mm, slub: detach percpu partial list in unfreeze_partials() using this_cpu_cmpxchg() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 26/34] mm, slub: only disable irq with spin_lock in __unfreeze_partials() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 27/34] mm, slub: don't disable irqs in slub_cpu_dead() Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 28/34] mm, slab: make flush_slab() possible to call with irqs enabled Vlastimil Babka
2021-06-09 11:38 ` [RFC v2 29/34] mm: slub: Move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context Vlastimil Babka
2021-06-09 22:29   ` Cyrill Gorcunov
2021-06-10  8:32     ` Vlastimil Babka
2021-06-10  8:36       ` Cyrill Gorcunov
2021-07-07  6:33   ` Hillf Danton
2021-06-09 11:38 ` [RFC v2 30/34] mm: slub: Make object_map_lock a raw_spinlock_t Vlastimil Babka
2021-06-09 11:39 ` [RFC v2 31/34] mm, slub: optionally save/restore irqs in slab_[un]lock()/ Vlastimil Babka
2021-07-02 12:17   ` Sebastian Andrzej Siewior
2021-06-09 11:39 ` [RFC v2 32/34] mm, slub: make slab_lock() disable irqs with PREEMPT_RT Vlastimil Babka
2021-06-09 11:39 ` [RFC v2 33/34] mm, slub: use migrate_disable() on PREEMPT_RT Vlastimil Babka
2021-06-14 11:07   ` Vlastimil Babka
2021-06-14 11:16     ` Sebastian Andrzej Siewior
2021-06-14 11:33       ` Vlastimil Babka
2021-06-14 12:54         ` Vlastimil Babka
2021-06-14 14:01         ` Sebastian Andrzej Siewior
2021-06-09 11:39 ` [RFC v2 34/34] mm, slub: convert kmem_cpu_slab protection to local_lock Vlastimil Babka
2021-06-14  9:49 ` [RFC v2 00/34] SLUB: reduce irq disabled scope and make it RT compatible Mel Gorman
2021-06-14 11:31   ` Mel Gorman
2021-06-14 11:10 ` Vlastimil Babka
2021-07-02 18:29 ` Sebastian Andrzej Siewior
2021-07-02 20:25   ` Vlastimil Babka
2021-07-29 13:49     ` Sebastian Andrzej Siewior [this message]
2021-07-29 14:17       ` Vlastimil Babka
2021-07-29 14:37         ` Sebastian Andrzej Siewior
2021-07-03  7:24   ` Mike Galbraith
2021-07-03 15:47     ` Mike Galbraith
2021-07-04  5:37       ` Mike Galbraith
2021-07-18  7:41     ` Vlastimil Babka
2021-07-18  8:29       ` Mike Galbraith
2021-07-18 12:09         ` Mike Galbraith
2021-07-05 16:00   ` Mike Galbraith
2021-07-06 17:56     ` Mike Galbraith

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