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

[Vlastimil Babka <vbabka@suse.cz>]

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.