linux-kernel - Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects

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Message-ID: <cf0fa2f4-86c3-655b-3a54-353bf8c1635b@linux.alibaba.com>
Date:   Fri, 31 Jul 2020 10:52:14 +0800
From:   xunlei <xlpang@...ux.alibaba.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Wen Yang <wenyang@...ux.alibaba.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial
 objects

On 2020/7/7 下午2:59, Christopher Lameter wrote:
> On Thu, 2 Jul 2020, Xunlei Pang wrote:
> 
>> This patch introduces two counters to maintain the actual number
>> of partial objects dynamically instead of iterating the partial
>> page lists with list_lock held.
>>
>> New counters of kmem_cache_node are: pfree_objects, ptotal_objects.
>> The main operations are under list_lock in slow path, its performance
>> impact is minimal.
> 
> 
> If at all then these counters need to be under CONFIG_SLUB_DEBUG.
> 
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -616,6 +616,8 @@ struct kmem_cache_node {
>>  #ifdef CONFIG_SLUB
>>  	unsigned long nr_partial;
>>  	struct list_head partial;
>> +	atomic_long_t pfree_objects; /* partial free objects */
>> +	atomic_long_t ptotal_objects; /* partial total objects */
> 
> Please in the CONFIG_SLUB_DEBUG. Without CONFIG_SLUB_DEBUG we need to
> build with minimal memory footprint.

Thanks for the comments.

show_slab_objects() also calls it with CONFIG_SYSFS:
    if (flags & SO_PARTIAL) {
        struct kmem_cache_node *n;

        for_each_kmem_cache_node(s, node, n) {
            if (flags & SO_TOTAL)
                x = count_partial(n, count_total);
            else if (flags & SO_OBJECTS)
                x = count_partial(n, count_inuse);
            else
                x = n->nr_partial;
            total += x;
            nodes[node] += x;
        }
    }

I'm not sure if it's due to some historical reason, it works without
CONFIG_SLUB_DEBUG.

> 
>>  #ifdef CONFIG_SLUB_DEBUG
>>  	atomic_long_t nr_slabs;
>>  	atomic_long_t total_objects;
>> diff --git a/mm/slub.c b/mm/slub.c
> 
> 
> 
> Also this looks to be quite heavy on the cache and on execution time. Note
> that the list_lock could be taken frequently in the performance sensitive
> case of freeing an object that is not in the partial lists.
> 

Yes, the concurrent __slab_free() has potential lock/atomic contention,
how about using percpu variable for partial free like below?
  static inline void
   __update_partial_free(struct kmem_cache_node *n, long delta)
  {
      atomic_long_add(delta, this_cpu_ptr(n->partial_free_objs));
  }

-Xunlei