Re: [patch 08/10] SLUB: Optional fast path using cmpxchg_local

Mathieu Desnoyers
2007-10-31T02:28:23Z

* Christoph Lameter (clameter@sgi.com) wrote:
> Provide an alternate implementation of the SLUB fast paths for alloc
> and free using cmpxchg_local. The cmpxchg_local fast path is selected
> for arches that have CONFIG_FAST_CMPXCHG_LOCAL set. An arch should only
> set CONFIG_FAST_CMPXCHG_LOCAL if the cmpxchg_local is faster than an
> interrupt enable/disable sequence. This is known to be true for both
> x86 platforms so set FAST_CMPXCHG_LOCAL for both arches.
>
> Not all arches can support fast cmpxchg operations. Typically the
> architecture must have an optimized cmpxchg instruction. The
> cmpxchg fast path makes no sense on platforms whose cmpxchg is
> slower than interrupt enable/disable (like f.e. IA64).
>
> The advantages of a cmpxchg_local based fast path are:
>
> 1. Lower cycle count (30%-60% faster)
>
> 2. There is no need to disable and enable interrupts on the fast path.
> Currently interrupts have to be disabled and enabled on every
> slab operation. This is likely saving a significant percentage
> of interrupt off / on sequences in the kernel.
>
> 3. The disposal of freed slabs can occur with interrupts enabled.
>

It would require some testing, but I suspect that powerpc, mips and m32r
are three other architectures that could benefit from this (from the top
of my head)

Mathieu

> The alternate path is realized using #ifdef's. Several attempts to do the
> same with macros and in line functions resulted in a mess (in particular due
> to the strange way that local_interrupt_save() handles its argument and due
> to the need to define macros/functions that sometimes disable interrupts
> and sometimes do something else. The macro based approaches made it also
> difficult to preserve the optimizations for the non cmpxchg paths).
>
> #ifdef seems to be the way to go here to have a readable source.
>
> Signed-off-by: Christoph Lameter <clameter@sgi.com>
>
> ---
> arch/x86/Kconfig.i386 | 4 ++
> arch/x86/Kconfig.x86_64 | 4 ++
> mm/slub.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++--
> 3 files changed, 77 insertions(+), 2 deletions(-)
>
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c 2007-10-27 10:39:07.583665939 -0700
> +++ linux-2.6/mm/slub.c 2007-10-27 10:40:19.710415861 -0700
> @@ -1496,7 +1496,12 @@ static void *__slab_alloc(struct kmem_ca
> {
> void **object;
> struct page *new;
> +#ifdef CONFIG_FAST_CMPXCHG_LOCAL
> + unsigned long flags;
>
> + local_irq_save(flags);
> + preempt_enable_no_resched();
> +#endif
> if (!c->page)
> goto new_slab;
>
> @@ -1518,6 +1523,10 @@ load_freelist:
> unlock_out:
> slab_unlock(c->page);
> out:
> +#ifdef CONFIG_FAST_CMPXCHG_LOCAL
> + preempt_disable();
> + local_irq_restore(flags);
> +#endif
> return object;
>
> another_slab:
> @@ -1592,9 +1601,26 @@ static void __always_inline *slab_alloc(
> gfp_t gfpflags, int node, void *addr)
> {
> void **object;
> - unsigned long flags;
> struct kmem_cache_cpu *c;
>
> +#ifdef CONFIG_FAST_CMPXCHG_LOCAL
> + c = get_cpu_slab(s, get_cpu());
> + do {
> + object = c->freelist;
> + if (unlikely(is_end(object) || !node_match(c, node))) {
> + object = __slab_alloc(s, gfpflags, node, addr, c);
> + if (unlikely(!object)) {
> + put_cpu();
> + goto out;
> + }
> + break;
> + }
> + } while (cmpxchg_local(&c->freelist, object, object[c->offset])
> + != object);
> + put_cpu();
> +#else
> + unsigned long flags;
> +
> local_irq_save(flags);
> c = get_cpu_slab(s, smp_processor_id());
> if (unlikely((is_end(c->freelist)) || !node_match(c, node))) {
> @@ -1609,6 +1635,7 @@ static void __always_inline *slab_alloc(
> c->freelist = object[c->offset];
> }
> local_irq_restore(flags);
> +#endif
>
> if (unlikely((gfpflags & __GFP_ZERO)))
> memset(object, 0, c->objsize);
> @@ -1644,6 +1671,11 @@ static void __slab_free(struct kmem_cach
> void *prior;
> void **object = (void *)x;
>
> +#ifdef CONFIG_FAST_CMPXCHG_LOCAL
> + unsigned long flags;
> +
> + local_irq_save(flags);
> +#endif
> slab_lock(page);
>
> if (unlikely(SlabDebug(page)))
> @@ -1669,6 +1701,9 @@ checks_ok:
>
> out_unlock:
> slab_unlock(page);
> +#ifdef CONFIG_FAST_CMPXCHG_LOCAL
> + local_irq_restore(flags);
> +#endif
> return;
>
> slab_empty:
> @@ -1679,6 +1714,9 @@ slab_empty:
> remove_partial(s, page);
>
> slab_unlock(page);
> +#ifdef CONFIG_FAST_CMPXCHG_LOCAL
> + local_irq_restore(flags);
> +#endif
> discard_slab(s, page);
> return;
>
> @@ -1703,9 +1741,37 @@ static void __always_inline slab_free(st
> struct page *page, void *x, void *addr)
> {
> void **object = (void *)x;
> - unsigned long flags;
> struct kmem_cache_cpu *c;
>
> +#ifdef CONFIG_FAST_CMPXCHG_LOCAL
> + void **freelist;
> +
> + c = get_cpu_slab(s, get_cpu());
> + debug_check_no_locks_freed(object, s->objsize);
> + do {
> + freelist = c->freelist;
> + barrier();
> + /*
> + * If the compiler would reorder the retrieval of c->page to
> + * come before c->freelist then an interrupt could
> + * change the cpu slab before we retrieve c->freelist. We
> + * could be matching on a page no longer active and put the
> + * object onto the freelist of the wrong slab.
> + *
> + * On the other hand: If we already have the freelist pointer
> + * then any change of cpu_slab will cause the cmpxchg to fail
> + * since the freelist pointers are unique per slab.
> + */
> + if (unlikely(page != c->page || c->node < 0)) {
> + __slab_free(s, page, x, addr, c->offset);
> + break;
> + }
> + object[c->offset] = freelist;
> + } while (cmpxchg_local(&c->freelist, freelist, object) != freelist);
> + put_cpu();
> +#else
> + unsigned long flags;
> +
> local_irq_save(flags);
> debug_check_no_locks_freed(object, s->objsize);
> c = get_cpu_slab(s, smp_processor_id());
> @@ -1716,6 +1782,7 @@ static void __always_inline slab_free(st
> __slab_free(s, page, x, addr, c->offset);
>
> local_irq_restore(flags);
> +#endif
> }
>
> void kmem_cache_free(struct kmem_cache *s, void *x)
> Index: linux-2.6/arch/x86/Kconfig.i386
> ===================================================================
> --- linux-2.6.orig/arch/x86/Kconfig.i386 2007-10-27 10:38:33.630415778 -0700
> +++ linux-2.6/arch/x86/Kconfig.i386 2007-10-27 10:40:19.710415861 -0700
> @@ -51,6 +51,10 @@ config X86
> bool
> default y
>
> +config FAST_CMPXCHG_LOCAL
> + bool
> + default y
> +
> config MMU
> bool
> default y
> Index: linux-2.6/arch/x86/Kconfig.x86_64
> ===================================================================
> --- linux-2.6.orig/arch/x86/Kconfig.x86_64 2007-10-27 10:38:33.630415778 -0700
> +++ linux-2.6/arch/x86/Kconfig.x86_64 2007-10-27 10:40:19.710415861 -0700
> @@ -97,6 +97,10 @@ config X86_CMPXCHG
> bool
> default y
>
> +config FAST_CMPXCHG_LOCAL
> + bool
> + default y
> +
> config EARLY_PRINTK
> bool
> default y
>
> --
> -
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--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68