Re: [RFC PATCH v3 7/7] slub: Optimize deactivate_slab()

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2023/10/31 19:15, Vlastimil Babka wrote:
> On 10/24/23 11:33, chengming.zhou@linux.dev wrote:
>> From: Chengming Zhou <zhouchengming@bytedance.com>
>>
>> Since the introduce of unfrozen slabs on cpu partial list, we don't
>> need to synchronize the slab frozen state under the node list_lock.
>>
>> The caller of deactivate_slab() and the caller of __slab_free() won't
>> manipulate the slab list concurrently.
>>
>> So we can get node list_lock in the last stage if we really need to
>> manipulate the slab list in this path.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> 
> Nice simplification!
> 
>> ---
>>  mm/slub.c | 70 ++++++++++++++++++++-----------------------------------
>>  1 file changed, 25 insertions(+), 45 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 486d44421432..64d550e415eb 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2449,10 +2449,8 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
>>  static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>>  			    void *freelist)
>>  {
>> -	enum slab_modes { M_NONE, M_PARTIAL, M_FREE, M_FULL_NOLIST };
>>  	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
>>  	int free_delta = 0;
>> -	enum slab_modes mode = M_NONE;
>>  	void *nextfree, *freelist_iter, *freelist_tail;
>>  	int tail = DEACTIVATE_TO_HEAD;
>>  	unsigned long flags = 0;
>> @@ -2499,58 +2497,40 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>>  	 * unfrozen and number of objects in the slab may have changed.
>>  	 * Then release lock and retry cmpxchg again.
>>  	 */
> 
> This comment above (including parts not visible here) should be updated as
> there is no more list manipulation during unfreeze.

Right!

> 
>> -redo:
>> -
>> -	old.freelist = READ_ONCE(slab->freelist);
>> -	old.counters = READ_ONCE(slab->counters);
>> -	VM_BUG_ON(!old.frozen);
>> -
>> -	/* Determine target state of the slab */
>> -	new.counters = old.counters;
>> -	if (freelist_tail) {
>> -		new.inuse -= free_delta;
>> -		set_freepointer(s, freelist_tail, old.freelist);
>> -		new.freelist = freelist;
>> -	} else
>> -		new.freelist = old.freelist;
>> +	do {
>> +		old.freelist = READ_ONCE(slab->freelist);
>> +		old.counters = READ_ONCE(slab->counters);
>> +		VM_BUG_ON(!old.frozen);
>> +
>> +		/* Determine target state of the slab */
>> +		new.counters = old.counters;
>> +		new.frozen = 0;
>> +		if (freelist_tail) {
>> +			new.inuse -= free_delta;
>> +			set_freepointer(s, freelist_tail, old.freelist);
>> +			new.freelist = freelist;
>> +		} else
>> +			new.freelist = old.freelist;
> 
> Per coding style we should have the else with { } even if it's one line, to
> match the if branch. Since we touch the code that was previously violating
> the style, we can fix up.

Ok, I will fix all these.

Big thanks for your review!

> 
>>  
>> -	new.frozen = 0;
>> +	} while (!slab_update_freelist(s, slab,
>> +		old.freelist, old.counters,
>> +		new.freelist, new.counters,
>> +		"unfreezing slab"));
>>  
>> +	/*
>> +	 * Stage three: Manipulate the slab list based on the updated state.
>> +	 */
>>  	if (!new.inuse && n->nr_partial >= s->min_partial) {
>> -		mode = M_FREE;
>> +		stat(s, DEACTIVATE_EMPTY);
>> +		discard_slab(s, slab);
>> +		stat(s, FREE_SLAB);
>>  	} else if (new.freelist) {
>> -		mode = M_PARTIAL;
>> -		/*
>> -		 * Taking the spinlock removes the possibility that
>> -		 * acquire_slab() will see a slab that is frozen
>> -		 */
>>  		spin_lock_irqsave(&n->list_lock, flags);
>> -	} else {
>> -		mode = M_FULL_NOLIST;
>> -	}
>> -
>> -
>> -	if (!slab_update_freelist(s, slab,
>> -				old.freelist, old.counters,
>> -				new.freelist, new.counters,
>> -				"unfreezing slab")) {
>> -		if (mode == M_PARTIAL)
>> -			spin_unlock_irqrestore(&n->list_lock, flags);
>> -		goto redo;
>> -	}
>> -
>> -
>> -	if (mode == M_PARTIAL) {
>>  		add_partial(n, slab, tail);
>>  		spin_unlock_irqrestore(&n->list_lock, flags);
>>  		stat(s, tail);
>> -	} else if (mode == M_FREE) {
>> -		stat(s, DEACTIVATE_EMPTY);
>> -		discard_slab(s, slab);
>> -		stat(s, FREE_SLAB);
>> -	} else if (mode == M_FULL_NOLIST) {
>> +	} else
>>  		stat(s, DEACTIVATE_FULL);
>> -	}
> 
> Same here.
> 
> Thanks!
> 
>>  }
>>  
>>  #ifdef CONFIG_SLUB_CPU_PARTIAL
>