Re: [PATCH rfc 2/4] percpu: split __pcpu_balance_workfn()

[Dennis Zhou <dennis@kernel.org>]

On Wed, Mar 24, 2021 at 12:06:24PM -0700, Roman Gushchin wrote:
> __pcpu_balance_workfn() became fairly big and hard to follow, but in
> fact it consists of two fully independent parts, responsible for
> the destruction of excessive free chunks and population of necessarily
> amount of free pages.
> 
> In order to simplify the code and prepare for adding of a new
> functionality, split it in two functions:
> 
>   1) pcpu_balance_free,
>   2) pcpu_balance_populated.
> 
> Move the taking/releasing of the pcpu_alloc_mutex to an upper level
> to keep the current synchronization in place.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
>  mm/percpu.c | 46 +++++++++++++++++++++++++++++-----------------
>  1 file changed, 29 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 78c55c73fa28..015d076893f5 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -1930,31 +1930,22 @@ void __percpu *__alloc_reserved_percpu(size_t size, size_t align)
>  }
>  
>  /**
> - * __pcpu_balance_workfn - manage the amount of free chunks and populated pages
> + * pcpu_balance_free - manage the amount of free chunks
>   * @type: chunk type
>   *
> - * Reclaim all fully free chunks except for the first one.  This is also
> - * responsible for maintaining the pool of empty populated pages.  However,
> - * it is possible that this is called when physical memory is scarce causing
> - * OOM killer to be triggered.  We should avoid doing so until an actual
> - * allocation causes the failure as it is possible that requests can be
> - * serviced from already backed regions.
> + * Reclaim all fully free chunks except for the first one.
>   */
> -static void __pcpu_balance_workfn(enum pcpu_chunk_type type)
> +static void pcpu_balance_free(enum pcpu_chunk_type type)
>  {
> -	/* gfp flags passed to underlying allocators */
> -	const gfp_t gfp = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
>  	LIST_HEAD(to_free);
>  	struct list_head *pcpu_slot = pcpu_chunk_list(type);
>  	struct list_head *free_head = &pcpu_slot[pcpu_nr_slots - 1];
>  	struct pcpu_chunk *chunk, *next;
> -	int slot, nr_to_pop, ret;
>  
>  	/*
>  	 * There's no reason to keep around multiple unused chunks and VM
>  	 * areas can be scarce.  Destroy all free chunks except for one.
>  	 */
> -	mutex_lock(&pcpu_alloc_mutex);
>  	spin_lock_irq(&pcpu_lock);
>  
>  	list_for_each_entry_safe(chunk, next, free_head, list) {
> @@ -1982,6 +1973,25 @@ static void __pcpu_balance_workfn(enum pcpu_chunk_type type)
>  		pcpu_destroy_chunk(chunk);
>  		cond_resched();
>  	}
> +}
> +
> +/**
> + * pcpu_balance_populated - manage the amount of populated pages
> + * @type: chunk type
> + *
> + * Maintain a certain amount of populated pages to satisfy atomic allocations.
> + * It is possible that this is called when physical memory is scarce causing
> + * OOM killer to be triggered.  We should avoid doing so until an actual
> + * allocation causes the failure as it is possible that requests can be
> + * serviced from already backed regions.
> + */
> +static void pcpu_balance_populated(enum pcpu_chunk_type type)
> +{
> +	/* gfp flags passed to underlying allocators */
> +	const gfp_t gfp = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
> +	struct list_head *pcpu_slot = pcpu_chunk_list(type);
> +	struct pcpu_chunk *chunk;
> +	int slot, nr_to_pop, ret;
>  
>  	/*
>  	 * Ensure there are certain number of free populated pages for
> @@ -2051,8 +2061,6 @@ static void __pcpu_balance_workfn(enum pcpu_chunk_type type)
>  			goto retry_pop;
>  		}
>  	}
> -
> -	mutex_unlock(&pcpu_alloc_mutex);
>  }
>  
>  /**
> @@ -2149,14 +2157,18 @@ static void pcpu_shrink_populated(enum pcpu_chunk_type type)
>   * pcpu_balance_workfn - manage the amount of free chunks and populated pages
>   * @work: unused
>   *
> - * Call __pcpu_balance_workfn() for each chunk type.
> + * Call pcpu_balance_free() and pcpu_balance_populated() for each chunk type.
>   */
>  static void pcpu_balance_workfn(struct work_struct *work)
>  {
>  	enum pcpu_chunk_type type;
>  
> -	for (type = 0; type < PCPU_NR_CHUNK_TYPES; type++)
> -		__pcpu_balance_workfn(type);
> +	for (type = 0; type < PCPU_NR_CHUNK_TYPES; type++) {
> +		mutex_lock(&pcpu_alloc_mutex);
> +		pcpu_balance_free(type);
> +		pcpu_balance_populated(type);
> +		mutex_unlock(&pcpu_alloc_mutex);
> +	}
>  }
>  
>  /**
> -- 
> 2.30.2
> 

Reviewed-by: Dennis Zhou <dennis@kernel.org>

This makes sense. If you want me to pick this and the last patch up
first I can. Otherwise, do you mind moving this to the front of the
stack because it is a clean up?

Thanks,
Dennis