Re: [patch 25/25] debugobjects: Track object usage to avoid premature freeing of objects

[Leizhen (ThunderTown)]

On 2024/10/8 0:50, Thomas Gleixner wrote:
> The freelist is freed at a constant rate independent of the actual usage
> requirements. That's bad in scenarios where usage comes in bursts. The end
> of a burst puts the object on the free list and freeing proceeds even when
> the next burst which requires objects started again.
> 
> Keep track of the usage with a exponentially wheighted moving average and
> take that into account in the worker function which frees objects from the
> free list.
> 
> This further reduces the kmem_cache allocation/free rate for a full kernel
> compile:
> 
>    	    kmem_cache_alloc()	kmem_cache_free()
> Baseline:   225k		245k
> Usage:	    170k		117k
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  lib/debugobjects.c |   62 ++++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 57 insertions(+), 5 deletions(-)
> 
> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -13,6 +13,7 @@
>  #include <linux/hash.h>
>  #include <linux/kmemleak.h>
>  #include <linux/sched.h>
> +#include <linux/sched/loadavg.h>
>  #include <linux/sched/task_stack.h>
>  #include <linux/seq_file.h>
>  #include <linux/slab.h>
> @@ -86,6 +87,7 @@ static struct obj_pool pool_to_free = {
>  
>  static HLIST_HEAD(pool_boot);
>  
> +static unsigned long		avg_usage;
>  static bool			obj_freeing;
>  
>  static int __data_racy			debug_objects_maxchain __read_mostly;
> @@ -382,11 +384,28 @@ static bool kmem_alloc_batch(struct hlis
>  	return true;
>  }
>  
> +static bool pool_can_fill(struct obj_pool *dst, struct obj_pool *src)
> +{
> +	unsigned int cnt = pool_count(dst);
> +
> +	if (cnt >= dst->min_cnt)
> +		return true;

There's already an interception in function debug_objects_fill_pool().
It's unlikely to be true.

debug_objects_fill_pool() --> fill_pool() --> pool_can_fill()
:
	if (likely(!pool_should_refill(&pool_global)))
		return;

> +
> +	return READ_ONCE(src->cnt) >= cnt - dst->min_cnt;

I don't understand. However, similar to above, fill_pool_from_freelist() has
been called before fill_pool() is called.

> +}
> +
>  static void fill_pool(void)
>  {
>  	static atomic_t cpus_allocating;
>  
>  	/*
> +	 * If the free pool has enough capacity, then don't try allocating
> +	 * unless the global pool has reached the cricital level.
> +	 */
> +	if (!pool_must_refill(&pool_global) && pool_can_fill(&pool_global, &pool_to_free))
> +		return;

As above, this code seems unnecessary.

> +
> +	/*
>  	 * Avoid allocation and lock contention when:
>  	 *   - One other CPU is already allocating
>  	 *   - the global pool has not reached the critical level yet
> @@ -427,11 +446,31 @@ static struct debug_obj *lookup_object(v
>  	return NULL;
>  }
>  
> +static void calc_usage(void)
> +{
> +	static DEFINE_RAW_SPINLOCK(avg_lock);
> +	static unsigned long avg_period;
> +	unsigned long cur, now = jiffies;
> +
> +	if (!time_after_eq(now, READ_ONCE(avg_period)))
> +		return;
> +
> +	if (!raw_spin_trylock(&avg_lock))
> +		return;
> +
> +	WRITE_ONCE(avg_period, now + msecs_to_jiffies(10));
> +	cur = READ_ONCE(pool_global.stats.cur_used) * ODEBUG_FREE_WORK_MAX;
> +	WRITE_ONCE(avg_usage, calc_load(avg_usage, EXP_5, cur));
> +	raw_spin_unlock(&avg_lock);
> +}
> +
>  static struct debug_obj *alloc_object(void *addr, struct debug_bucket *b,
>  				      const struct debug_obj_descr *descr)
>  {
>  	struct debug_obj *obj;
>  
> +	calc_usage();
> +
>  	if (static_branch_likely(&obj_cache_enabled))
>  		obj = pcpu_alloc();
>  	else
> @@ -450,14 +489,26 @@ static struct debug_obj *alloc_object(vo
>  /* workqueue function to free objects. */
>  static void free_obj_work(struct work_struct *work)
>  {
> -	bool free = true;
> +	static unsigned long last_use_avg;
> +	unsigned long cur_used, last_used, delta;
> +	unsigned int max_free = 0;
>  
>  	WRITE_ONCE(obj_freeing, false);
>  
> +	/* Rate limit freeing based on current use average */
> +	cur_used = READ_ONCE(avg_usage);
> +	last_used = last_use_avg;
> +	last_use_avg = cur_used;
> +
>  	if (!pool_count(&pool_to_free))
>  		return;
>  
> -	for (unsigned int cnt = 0; cnt < ODEBUG_FREE_WORK_MAX; cnt++) {
> +	if (cur_used <= last_used) {
> +		delta = (last_used - cur_used) / ODEBUG_FREE_WORK_MAX;
> +		max_free = min(delta, ODEBUG_FREE_WORK_MAX);
> +	}
> +
> +	for (int cnt = 0; cnt < ODEBUG_FREE_WORK_MAX; cnt++) {
>  		HLIST_HEAD(tofree);
>  
>  		/* Acquire and drop the lock for each batch */
> @@ -468,9 +519,10 @@ static void free_obj_work(struct work_st
>  			/* Refill the global pool if possible */
>  			if (pool_move_batch(&pool_global, &pool_to_free)) {
>  				/* Don't free as there seems to be demand */
> -				free = false;
> -			} else if (free) {
> +				max_free = 0;
> +			} else if (max_free) {
>  				pool_pop_batch(&tofree, &pool_to_free);
> +				max_free--;
>  			} else {
>  				return;
>  			}
> @@ -1110,7 +1162,7 @@ static int debug_stats_show(struct seq_f
>  	for_each_possible_cpu(cpu)
>  		pcp_free += per_cpu(pool_pcpu.cnt, cpu);
>  
> -	pool_used = data_race(pool_global.stats.cur_used);
> +	pool_used = READ_ONCE(pool_global.stats.cur_used);
>  	pcp_free = min(pool_used, pcp_free);
>  	pool_used -= pcp_free;
>  
> 
> .
> 

-- 
Regards,
  Zhen Lei