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Harry Yoo (Oracle)
Apr 13, 2026, 3:59:04 AMApr 13
to Thomas Gleixner, LKML, Vlastimil Babka, linu...@kvack.org, Arnd Bergmann, x...@kernel.org, Lu Baolu, io...@lists.linux.dev, Michael Grzeschik, net...@vger.kernel.org, linux-w...@vger.kernel.org, Herbert Xu, linux-...@vger.kernel.org, David Woodhouse, Bernie Thompson, linux...@vger.kernel.org, Theodore Tso, linux...@vger.kernel.org, Andrew Morton, Uladzislau Rezki, Marco Elver, Dmitry Vyukov, kasa...@googlegroups.com, Andrey Ryabinin, Thomas Sailer, linux...@vger.kernel.org, Jason A. Donenfeld, Richard Henderson, linux...@vger.kernel.org, Russell King, linux-ar...@lists.infradead.org, Catalin Marinas, Huacai Chen, loon...@lists.linux.dev, Geert Uytterhoeven, linux...@lists.linux-m68k.org, Dinh Nguyen, Jonas Bonn, linux-o...@vger.kernel.org, Helge Deller, linux-...@vger.kernel.org, Michael Ellerman, linuxp...@lists.ozlabs.org, Paul Walmsley, linux...@lists.infradead.org, Heiko Carstens, linux...@vger.kernel.org, David S. Miller, sparc...@vger.kernel.org, Hao Li, Christoph Lameter, David Rientjes, Roman Gushchin, Shengming Hu
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Subject: Re: [patch 14/38] slub: Use prandom instead of get_cycles()
Reply-To:
In-Reply-To: <202604101203...@kernel.org>
On Fri, Apr 10, 2026 at 02:19:37PM +0200, Thomas Gleixner wrote:
> The decision whether to scan remote nodes is based on a 'random' number
> retrieved via get_cycles(). get_cycles() is about to be removed.
>
> There is already prandom state in the code, so use that instead.
>
> Signed-off-by: Thomas Gleixner <tg...@kernel.org>
> Cc: Vlastimil Babka <vba...@kernel.org>
> Cc: linu...@kvack.org
> ---
Acked-by: Harry Yoo (Oracle) <ha...@kernel.org>
Is this for this merge window?
This may conflict with upcoming changes on freelist shuffling [1]
(not queued for slab/for-next yet though), but it should be easy to
resolve.
[Cc'ing Shengming and SLAB ALLOCATOR folks]
[1] https://lore.kernel.org/linux-mm/20260409204352095kK...@zte.com.cn
--
Cheers,
Harry / Hyeonggon
> mm/slub.c | 37 +++++++++++++++++++++++--------------
> 1 file changed, 23 insertions(+), 14 deletions(-)
>
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3302,6 +3302,25 @@ static inline struct slab *alloc_slab_pa
> return slab;
> }
>
> +#if defined(CONFIG_SLAB_FREELIST_RANDOM) || defined(CONFIG_NUMA)
> +static DEFINE_PER_CPU(struct rnd_state, slab_rnd_state);
> +
> +static unsigned int slab_get_prandom_state(unsigned int limit)
> +{
> + struct rnd_state *state;
> + unsigned int res;
> +
> + /*
> + * An interrupt or NMI handler might interrupt and change
> + * the state in the middle, but that's safe.
> + */
> + state = &get_cpu_var(slab_rnd_state);
> + res = prandom_u32_state(state) % limit;
> + put_cpu_var(slab_rnd_state);
> + return res;
> +}
> +#endif
> +
> #ifdef CONFIG_SLAB_FREELIST_RANDOM
> /* Pre-initialize the random sequence cache */
> static int init_cache_random_seq(struct kmem_cache *s)
> @@ -3365,8 +3384,6 @@ static void *next_freelist_entry(struct
> return (char *)start + idx;
> }
>
> -static DEFINE_PER_CPU(struct rnd_state, slab_rnd_state);
> -
> /* Shuffle the single linked freelist based on a random pre-computed sequence */
> static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab,
> bool allow_spin)
> @@ -3383,15 +3400,7 @@ static bool shuffle_freelist(struct kmem
> if (allow_spin) {
> pos = get_random_u32_below(freelist_count);
> } else {
> - struct rnd_state *state;
> -
> - /*
> - * An interrupt or NMI handler might interrupt and change
> - * the state in the middle, but that's safe.
> - */
> - state = &get_cpu_var(slab_rnd_state);
> - pos = prandom_u32_state(state) % freelist_count;
> - put_cpu_var(slab_rnd_state);
> + pos = slab_get_prandom_state(freelist_count);
> }
>
> page_limit = slab->objects * s->size;
> @@ -3882,7 +3891,7 @@ static void *get_from_any_partial(struct
> * with available objects.
> */
> if (!s->remote_node_defrag_ratio ||
> - get_cycles() % 1024 > s->remote_node_defrag_ratio)
> + slab_get_prandom_state(1024) > s->remote_node_defrag_ratio)
> return NULL;
>
> do {
> @@ -7102,7 +7111,7 @@ static unsigned int
>
> /* see get_from_any_partial() for the defrag ratio description */
> if (!s->remote_node_defrag_ratio ||
> - get_cycles() % 1024 > s->remote_node_defrag_ratio)
> + slab_get_prandom_state(1024) > s->remote_node_defrag_ratio)
> return 0;
>
> do {
> @@ -8421,7 +8430,7 @@ void __init kmem_cache_init_late(void)
> flushwq = alloc_workqueue("slub_flushwq", WQ_MEM_RECLAIM | WQ_PERCPU,
> 0);
> WARN_ON(!flushwq);
> -#ifdef CONFIG_SLAB_FREELIST_RANDOM
> +#if defined(CONFIG_SLAB_FREELIST_RANDOM) || defined(CONFIG_NUMA)
> prandom_init_once(&slab_rnd_state);
> #endif
> }
>
>
Subject: Re: [patch 14/38] slub: Use prandom instead of get_cycles()
Reply-To:
In-Reply-To: <202604101203...@kernel.org>
On Fri, Apr 10, 2026 at 02:19:37PM +0200, Thomas Gleixner wrote:
> The decision whether to scan remote nodes is based on a 'random' number
> retrieved via get_cycles(). get_cycles() is about to be removed.
>
> There is already prandom state in the code, so use that instead.
>
> Signed-off-by: Thomas Gleixner <tg...@kernel.org>
> Cc: Vlastimil Babka <vba...@kernel.org>
> Cc: linu...@kvack.org
> ---
Acked-by: Harry Yoo (Oracle) <ha...@kernel.org>
Is this for this merge window?
This may conflict with upcoming changes on freelist shuffling [1]
(not queued for slab/for-next yet though), but it should be easy to
resolve.
[Cc'ing Shengming and SLAB ALLOCATOR folks]
[1] https://lore.kernel.org/linux-mm/20260409204352095kK...@zte.com.cn
--
Cheers,
Harry / Hyeonggon
> mm/slub.c | 37 +++++++++++++++++++++++--------------
> 1 file changed, 23 insertions(+), 14 deletions(-)
>
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3302,6 +3302,25 @@ static inline struct slab *alloc_slab_pa
> return slab;
> }
>
> +#if defined(CONFIG_SLAB_FREELIST_RANDOM) || defined(CONFIG_NUMA)
> +static DEFINE_PER_CPU(struct rnd_state, slab_rnd_state);
> +
> +static unsigned int slab_get_prandom_state(unsigned int limit)
> +{
> + struct rnd_state *state;
> + unsigned int res;
> +
> + /*
> + * An interrupt or NMI handler might interrupt and change
> + * the state in the middle, but that's safe.
> + */
> + state = &get_cpu_var(slab_rnd_state);
> + res = prandom_u32_state(state) % limit;
> + put_cpu_var(slab_rnd_state);
> + return res;
> +}
> +#endif
> +
> #ifdef CONFIG_SLAB_FREELIST_RANDOM
> /* Pre-initialize the random sequence cache */
> static int init_cache_random_seq(struct kmem_cache *s)
> @@ -3365,8 +3384,6 @@ static void *next_freelist_entry(struct
> return (char *)start + idx;
> }
>
> -static DEFINE_PER_CPU(struct rnd_state, slab_rnd_state);
> -
> /* Shuffle the single linked freelist based on a random pre-computed sequence */
> static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab,
> bool allow_spin)
> @@ -3383,15 +3400,7 @@ static bool shuffle_freelist(struct kmem
> if (allow_spin) {
> pos = get_random_u32_below(freelist_count);
> } else {
> - struct rnd_state *state;
> -
> - /*
> - * An interrupt or NMI handler might interrupt and change
> - * the state in the middle, but that's safe.
> - */
> - state = &get_cpu_var(slab_rnd_state);
> - pos = prandom_u32_state(state) % freelist_count;
> - put_cpu_var(slab_rnd_state);
> + pos = slab_get_prandom_state(freelist_count);
> }
>
> page_limit = slab->objects * s->size;
> @@ -3882,7 +3891,7 @@ static void *get_from_any_partial(struct
> * with available objects.
> */
> if (!s->remote_node_defrag_ratio ||
> - get_cycles() % 1024 > s->remote_node_defrag_ratio)
> + slab_get_prandom_state(1024) > s->remote_node_defrag_ratio)
> return NULL;
>
> do {
> @@ -7102,7 +7111,7 @@ static unsigned int
>
> /* see get_from_any_partial() for the defrag ratio description */
> if (!s->remote_node_defrag_ratio ||
> - get_cycles() % 1024 > s->remote_node_defrag_ratio)
> + slab_get_prandom_state(1024) > s->remote_node_defrag_ratio)
> return 0;
>
> do {
> @@ -8421,7 +8430,7 @@ void __init kmem_cache_init_late(void)
> flushwq = alloc_workqueue("slub_flushwq", WQ_MEM_RECLAIM | WQ_PERCPU,
> 0);
> WARN_ON(!flushwq);
> -#ifdef CONFIG_SLAB_FREELIST_RANDOM
> +#if defined(CONFIG_SLAB_FREELIST_RANDOM) || defined(CONFIG_NUMA)
> prandom_init_once(&slab_rnd_state);
> #endif
> }
>
>
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