Re: [PATCH v3 08/11] slub: Replace cmpxchg_double()

Vlastimil Babka
2023-05-24T10:13:19Z

On 5/24/23 11:32, Peter Zijlstra wrote:
> On Mon, May 15, 2023 at 09:57:07AM +0200, Peter Zijlstra wrote:
>
>> @@ -3008,6 +3029,22 @@ static inline bool pfmemalloc_match(stru
>> }
>>
>> #ifndef CONFIG_SLUB_TINY
>> +static inline bool
>> +__update_cpu_freelist_fast(struct kmem_cache *s,
>> + void *freelist_old, void *freelist_new,
>> + unsigned long tid)
>> +{
>> +#ifdef system_has_freelist_aba
>> + freelist_aba_t old = { .freelist = freelist_old, .counter = tid };
>> + freelist_aba_t new = { .freelist = freelist_new, .counter = next_tid(tid) };
>> +
>> + return this_cpu_cmpxchg_freelist(s->cpu_slab->freelist_tid.full,
>> + old.full, new.full) == old.full;
>> +#else
>> + return false;
>> +#endif
>> +}
>> +
>> /*
>> * Check the slab->freelist and either transfer the freelist to the
>> * per cpu freelist or deactivate the slab.
>> @@ -3359,11 +3396,7 @@ static __always_inline void *__slab_allo
>> * against code executing on this cpu *not* from access by
>> * other cpus.
>> */
>> - if (unlikely(!this_cpu_cmpxchg_double(
>> - s->cpu_slab->freelist, s->cpu_slab->tid,
>> - object, tid,
>> - next_object, next_tid(tid)))) {
>> -
>> + if (unlikely(!__update_cpu_freelist_fast(s, object, next_object, tid))) {
>> note_cmpxchg_failure("slab_alloc", s, tid);
>> goto redo;
>> }
>> @@ -3736,11 +3769,7 @@ static __always_inline void do_slab_free
>>
>> set_freepointer(s, tail_obj, freelist);
>>
>> - if (unlikely(!this_cpu_cmpxchg_double(
>> - s->cpu_slab->freelist, s->cpu_slab->tid,
>> - freelist, tid,
>> - head, next_tid(tid)))) {
>> -
>> + if (unlikely(!__update_cpu_freelist_fast(s, freelist, head, tid))) {
>> note_cmpxchg_failure("slab_free", s, tid);
>> goto redo;
>> }
>
> This isn't right; the this_cpu_cmpxchg_double() was unconditional and
> relied on the local_irq_save() fallback when no native cmpxchg128 is
> present.
>
> The below delta makes things boot again when system_has_cmpxchg128 is
> not defined.

Right, that should do.

> I'm going to zap these patches from tip/locking/core for a few days and
> fold the below back into the series and let it run through the robots
> again.

I noticed some comments in mm/slub.c still mention "cmpxchg_double", dunno
how much you want to clean it right now or can be postponed. Also some sysfs
stats files for CONFIG_SLUB_STATS (not widely used) which we probably might
try renaming without breaking anyone, but it's not guaranteed.