Re: [patch 14/38] slub: Use prandom instead of get_cycles()

hu.shengming
2026-04-13T13:03:09Z

Harry wrote:
> [Resending after fixing broken email headers]
>
> 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 <tglx@kernel.org>
> > Cc: Vlastimil Babka <vbabka@kernel.org>
> > Cc: linux-mm@kvack.org
> > ---
>
> Acked-by: Harry Yoo (Oracle) <harry@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.
>

Hi Harry,

Would you like me to wait for this patch to land linux-next and then
rebase and send v6 on top?

Thanks,

--
With Best Regards,
Shengming

> [Cc'ing Shengming and SLAB ALLOCATOR folks]
> [1] https://lore.kernel.org/linux-mm/20260409204352095kKWVYKtZImN59ybO6iRNj@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
> > }
> >
> >