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

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 <[email protected]>
> > Cc: Vlastimil Babka <[email protected]>
> > Cc: [email protected]
> > ---
> 
> Acked-by: Harry Yoo (Oracle) <[email protected]>
> 
> 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/[email protected]
> 
> -- 
> 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
> >  }
> > 
> >