The patch titled
Subject: mm, slub: simplify kmem_cache_cpu and tid setup
has been added to the -mm tree. Its filename is
mm-slub-simplify-kmem_cache_cpu-and-tid-setup.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-slub-simplify-kmem_cache_cpu-and-tid-setup.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-slub-simplify-kmem_cache_cpu-and-tid-setup.patch
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From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: mm, slub: simplify kmem_cache_cpu and tid setup
In slab_alloc_node() and do_slab_free() fastpaths we need to guarantee
that our kmem_cache_cpu pointer is from the same cpu as the tid value.
Currently that's done by reading the tid first using this_cpu_read(), then
the kmem_cache_cpu pointer and verifying we read the same tid using the
pointer and plain READ_ONCE().
This can be simplified to just fetching kmem_cache_cpu pointer and then
reading tid using the pointer. That guarantees they are from the same
cpu. We don't need to read the tid using this_cpu_read() because the
value will be validated by this_cpu_cmpxchg_double(), making sure we are
on the correct cpu and the freelist didn't change by anyone preempting us
since reading the tid.
Link: https://lkml.kernel.org/r/20210805152000.12817-12-vbabka@xxxxxxx
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/slub.c | 22 +++++++++-------------
1 file changed, 9 insertions(+), 13 deletions(-)
--- a/mm/slub.c~mm-slub-simplify-kmem_cache_cpu-and-tid-setup
+++ a/mm/slub.c
@@ -2868,15 +2868,14 @@ redo:
* reading from one cpu area. That does not matter as long
* as we end up on the original cpu again when doing the cmpxchg.
*
- * We should guarantee that tid and kmem_cache are retrieved on
- * the same cpu. It could be different if CONFIG_PREEMPTION so we need
- * to check if it is matched or not.
+ * We must guarantee that tid and kmem_cache_cpu are retrieved on the
+ * same cpu. We read first the kmem_cache_cpu pointer and use it to read
+ * the tid. If we are preempted and switched to another cpu between the
+ * two reads, it's OK as the two are still associated with the same cpu
+ * and cmpxchg later will validate the cpu.
*/
- do {
- tid = this_cpu_read(s->cpu_slab->tid);
- c = raw_cpu_ptr(s->cpu_slab);
- } while (IS_ENABLED(CONFIG_PREEMPTION) &&
- unlikely(tid != READ_ONCE(c->tid)));
+ c = raw_cpu_ptr(s->cpu_slab);
+ tid = READ_ONCE(c->tid);
/*
* Irqless object alloc/free algorithm used here depends on sequence
@@ -3150,11 +3149,8 @@ redo:
* data is retrieved via this pointer. If we are on the same cpu
* during the cmpxchg then the free will succeed.
*/
- do {
- tid = this_cpu_read(s->cpu_slab->tid);
- c = raw_cpu_ptr(s->cpu_slab);
- } while (IS_ENABLED(CONFIG_PREEMPTION) &&
- unlikely(tid != READ_ONCE(c->tid)));
+ c = raw_cpu_ptr(s->cpu_slab);
+ tid = READ_ONCE(c->tid);
/* Same with comment on barrier() in slab_alloc_node() */
barrier();
_
Patches currently in -mm which might be from vbabka@xxxxxxx are
mm-slub-fix-slub_debug-disablement-for-list-of-slabs.patch
mm-slub-dont-call-flush_all-from-slab_debug_trace_open.patch
mm-slub-allocate-private-object-map-for-debugfs-listings.patch
mm-slub-allocate-private-object-map-for-validate_slab_cache.patch
mm-slub-dont-disable-irq-for-debug_check_no_locks_freed.patch
mm-slub-remove-redundant-unfreeze_partials-from-put_cpu_partial.patch
mm-slub-unify-cmpxchg_double_slab-and-__cmpxchg_double_slab.patch
mm-slub-extract-get_partial-from-new_slab_objects.patch
mm-slub-dissolve-new_slab_objects-into-___slab_alloc.patch
mm-slub-return-slab-page-from-get_partial-and-set-c-page-afterwards.patch
mm-slub-restructure-new-page-checks-in-___slab_alloc.patch
mm-slub-simplify-kmem_cache_cpu-and-tid-setup.patch
mm-slub-move-disabling-enabling-irqs-to-___slab_alloc.patch
mm-slub-do-initial-checks-in-___slab_alloc-with-irqs-enabled.patch
mm-slub-move-disabling-irqs-closer-to-get_partial-in-___slab_alloc.patch
mm-slub-restore-irqs-around-calling-new_slab.patch
mm-slub-validate-slab-from-partial-list-or-page-allocator-before-making-it-cpu-slab.patch
mm-slub-check-new-pages-with-restored-irqs.patch
mm-slub-stop-disabling-irqs-around-get_partial.patch
mm-slub-move-reset-of-c-page-and-freelist-out-of-deactivate_slab.patch
mm-slub-make-locking-in-deactivate_slab-irq-safe.patch
mm-slub-call-deactivate_slab-without-disabling-irqs.patch
mm-slub-move-irq-control-into-unfreeze_partials.patch
mm-slub-discard-slabs-in-unfreeze_partials-without-irqs-disabled.patch
mm-slub-detach-whole-partial-list-at-once-in-unfreeze_partials.patch
mm-slub-separate-detaching-of-partial-list-in-unfreeze_partials-from-unfreezing.patch
mm-slub-only-disable-irq-with-spin_lock-in-__unfreeze_partials.patch
mm-slub-dont-disable-irqs-in-slub_cpu_dead.patch
mm-slab-make-flush_slab-possible-to-call-with-irqs-enabled.patch
mm-slub-optionally-save-restore-irqs-in-slab_lock.patch
mm-slub-make-slab_lock-disable-irqs-with-preempt_rt.patch
mm-slub-protect-put_cpu_partial-with-disabled-irqs-instead-of-cmpxchg.patch
mm-slub-use-migrate_disable-on-preempt_rt.patch
mm-slub-convert-kmem_cpu_slab-protection-to-local_lock.patch
