Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

[Frank Rowand]

+Chintan

Hi Chintan,

On 12/11/19 5:23 PM, Rob Herring wrote:
> The phandle cache was added to speed up of_find_node_by_phandle() by
> avoiding walking the whole DT to find a matching phandle. The
> implementation has several shortcomings:
> 
>   - The cache is designed to work on a linear set of phandle values.
>     This is true for dtc generated DTs, but not for other cases such as
>     Power.
>   - The cache isn't enabled until of_core_init() and a typical system
>     may see hundreds of calls to of_find_node_by_phandle() before that
>     point.
>   - The cache is freed and re-allocated when the number of phandles
>     changes.
>   - It takes a raw spinlock around a memory allocation which breaks on
>     RT.
> 
> Change the implementation to a fixed size and use hash_32() as the
> cache index. This greatly simplifies the implementation. It avoids
> the need for any re-alloc of the cache and taking a reference on nodes
> in the cache. We only have a single source of removing cache entries
> which is of_detach_node().

Can you check this patch on the system that you reported phandle lookup
overhead issues for?

-Frank


> 
> Using hash_32() removes any assumption on phandle values improving
> the hit rate for non-linear phandle values. The effect on linear values
> using hash_32() is about a 10% collision. The chances of thrashing on
> colliding values seems to be low.
> 
> To compare performance, I used a RK3399 board which is a pretty typical
> system. I found that just measuring boot time as done previously is
> noisy and may be impacted by other things. Also bringing up secondary
> cores causes some issues with measuring, so I booted with 'nr_cpus=1'.
> With no caching, calls to of_find_node_by_phandle() take about 20124 us
> for 1248 calls. There's an additional 288 calls before time keeping is
> up. Using the average time per hit/miss with the cache, we can calculate
> these calls to take 690 us (277 hit / 11 miss) with a 128 entry cache
> and 13319 us with no cache or an uninitialized cache.
> 
> Comparing the 3 implementations the time spent in
> of_find_node_by_phandle() is:
> 
> no cache:        20124 us (+ 13319 us)
> 128 entry cache:  5134 us (+ 690 us)
> current cache:     819 us (+ 13319 us)
> 
> We could move the allocation of the cache earlier to improve the
> current cache, but that just further complicates the situation as it
> needs to be after slab is up, so we can't do it when unflattening (which
> uses memblock).
> 
> Reported-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>
> Cc: Frank Rowand <frowand.list@xxxxxxxxx>
> Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
> ---
>  drivers/of/base.c       | 133 ++++++++--------------------------------
>  drivers/of/dynamic.c    |   2 +-
>  drivers/of/of_private.h |   4 +-
>  drivers/of/overlay.c    |  10 ---
>  4 files changed, 28 insertions(+), 121 deletions(-)
> 
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index db7fbc0c0893..f7da162f126d 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -135,115 +135,38 @@ int __weak of_node_to_nid(struct device_node *np)
>  }
>  #endif
>  
> -/*
> - * Assumptions behind phandle_cache implementation:
> - *   - phandle property values are in a contiguous range of 1..n
> - *
> - * If the assumptions do not hold, then
> - *   - the phandle lookup overhead reduction provided by the cache
> - *     will likely be less
> - */
> +#define OF_PHANDLE_CACHE_BITS	7
> +#define OF_PHANDLE_CACHE_SZ	BIT(OF_PHANDLE_CACHE_BITS)
>  
> -static struct device_node **phandle_cache;
> -static u32 phandle_cache_mask;
> +static struct device_node *phandle_cache[OF_PHANDLE_CACHE_SZ];
>  
> -/*
> - * Caller must hold devtree_lock.
> - */
> -static void __of_free_phandle_cache(void)
> +static u32 of_phandle_cache_hash(phandle handle)
>  {
> -	u32 cache_entries = phandle_cache_mask + 1;
> -	u32 k;
> -
> -	if (!phandle_cache)
> -		return;
> -
> -	for (k = 0; k < cache_entries; k++)
> -		of_node_put(phandle_cache[k]);
> -
> -	kfree(phandle_cache);
> -	phandle_cache = NULL;
> +	return hash_32(handle, OF_PHANDLE_CACHE_BITS);
>  }
>  
> -int of_free_phandle_cache(void)
> -{
> -	unsigned long flags;
> -
> -	raw_spin_lock_irqsave(&devtree_lock, flags);
> -
> -	__of_free_phandle_cache();
> -
> -	raw_spin_unlock_irqrestore(&devtree_lock, flags);
> -
> -	return 0;
> -}
> -#if !defined(CONFIG_MODULES)
> -late_initcall_sync(of_free_phandle_cache);
> -#endif
> -
>  /*
>   * Caller must hold devtree_lock.
>   */
> -void __of_free_phandle_cache_entry(phandle handle)
> +void __of_phandle_cache_inv_entry(phandle handle)
>  {
> -	phandle masked_handle;
> +	u32 handle_hash;
>  	struct device_node *np;
>  
>  	if (!handle)
>  		return;
>  
> -	masked_handle = handle & phandle_cache_mask;
> +	handle_hash = of_phandle_cache_hash(handle);
>  
> -	if (phandle_cache) {
> -		np = phandle_cache[masked_handle];
> -		if (np && handle == np->phandle) {
> -			of_node_put(np);
> -			phandle_cache[masked_handle] = NULL;
> -		}
> -	}
> -}
> -
> -void of_populate_phandle_cache(void)
> -{
> -	unsigned long flags;
> -	u32 cache_entries;
> -	struct device_node *np;
> -	u32 phandles = 0;
> -
> -	raw_spin_lock_irqsave(&devtree_lock, flags);
> -
> -	__of_free_phandle_cache();
> -
> -	for_each_of_allnodes(np)
> -		if (np->phandle && np->phandle != OF_PHANDLE_ILLEGAL)
> -			phandles++;
> -
> -	if (!phandles)
> -		goto out;
> -
> -	cache_entries = roundup_pow_of_two(phandles);
> -	phandle_cache_mask = cache_entries - 1;
> -
> -	phandle_cache = kcalloc(cache_entries, sizeof(*phandle_cache),
> -				GFP_ATOMIC);
> -	if (!phandle_cache)
> -		goto out;
> -
> -	for_each_of_allnodes(np)
> -		if (np->phandle && np->phandle != OF_PHANDLE_ILLEGAL) {
> -			of_node_get(np);
> -			phandle_cache[np->phandle & phandle_cache_mask] = np;
> -		}
> -
> -out:
> -	raw_spin_unlock_irqrestore(&devtree_lock, flags);
> +	np = phandle_cache[handle_hash];
> +	if (np && handle == np->phandle)
> +		phandle_cache[handle_hash] = NULL;
>  }
>  
>  void __init of_core_init(void)
>  {
>  	struct device_node *np;
>  
> -	of_populate_phandle_cache();
>  
>  	/* Create the kset, and register existing nodes */
>  	mutex_lock(&of_mutex);
> @@ -253,8 +176,11 @@ void __init of_core_init(void)
>  		pr_err("failed to register existing nodes\n");
>  		return;
>  	}
> -	for_each_of_allnodes(np)
> +	for_each_of_allnodes(np) {
>  		__of_attach_node_sysfs(np);
> +		if (np->phandle && !phandle_cache[of_phandle_cache_hash(np->phandle)])
> +			phandle_cache[of_phandle_cache_hash(np->phandle)] = np;
> +	}
>  	mutex_unlock(&of_mutex);
>  
>  	/* Symlink in /proc as required by userspace ABI */
> @@ -1235,36 +1161,29 @@ struct device_node *of_find_node_by_phandle(phandle handle)
>  {
>  	struct device_node *np = NULL;
>  	unsigned long flags;
> -	phandle masked_handle;
> +	u32 handle_hash;
>  
>  	if (!handle)
>  		return NULL;
>  
> +	handle_hash = of_phandle_cache_hash(handle);
> +
>  	raw_spin_lock_irqsave(&devtree_lock, flags);
>  
> -	masked_handle = handle & phandle_cache_mask;
> -
> -	if (phandle_cache) {
> -		if (phandle_cache[masked_handle] &&
> -		    handle == phandle_cache[masked_handle]->phandle)
> -			np = phandle_cache[masked_handle];
> -		if (np && of_node_check_flag(np, OF_DETACHED)) {
> -			WARN_ON(1); /* did not uncache np on node removal */
> -			of_node_put(np);
> -			phandle_cache[masked_handle] = NULL;
> -			np = NULL;
> -		}
> +	if (phandle_cache[handle_hash] &&
> +	    handle == phandle_cache[handle_hash]->phandle)
> +		np = phandle_cache[handle_hash];
> +	if (np && of_node_check_flag(np, OF_DETACHED)) {
> +		WARN_ON(1); /* did not uncache np on node removal */
> +		phandle_cache[handle_hash] = NULL;
> +		np = NULL;
>  	}
>  
>  	if (!np) {
>  		for_each_of_allnodes(np)
>  			if (np->phandle == handle &&
>  			    !of_node_check_flag(np, OF_DETACHED)) {
> -				if (phandle_cache) {
> -					/* will put when removed from cache */
> -					of_node_get(np);
> -					phandle_cache[masked_handle] = np;
> -				}
> +				phandle_cache[handle_hash] = np;
>  				break;
>  			}
>  	}
> diff --git a/drivers/of/dynamic.c b/drivers/of/dynamic.c
> index 49b16f76d78e..08fd823edac9 100644
> --- a/drivers/of/dynamic.c
> +++ b/drivers/of/dynamic.c
> @@ -276,7 +276,7 @@ void __of_detach_node(struct device_node *np)
>  	of_node_set_flag(np, OF_DETACHED);
>  
>  	/* race with of_find_node_by_phandle() prevented by devtree_lock */
> -	__of_free_phandle_cache_entry(np->phandle);
> +	__of_phandle_cache_inv_entry(np->phandle);
>  }
>  
>  /**
> diff --git a/drivers/of/of_private.h b/drivers/of/of_private.h
> index 66294d29942a..582844c158ae 100644
> --- a/drivers/of/of_private.h
> +++ b/drivers/of/of_private.h
> @@ -85,14 +85,12 @@ int of_resolve_phandles(struct device_node *tree);
>  #endif
>  
>  #if defined(CONFIG_OF_DYNAMIC)
> -void __of_free_phandle_cache_entry(phandle handle);
> +void __of_phandle_cache_inv_entry(phandle handle);
>  #endif
>  
>  #if defined(CONFIG_OF_OVERLAY)
>  void of_overlay_mutex_lock(void);
>  void of_overlay_mutex_unlock(void);
> -int of_free_phandle_cache(void);
> -void of_populate_phandle_cache(void);
>  #else
>  static inline void of_overlay_mutex_lock(void) {};
>  static inline void of_overlay_mutex_unlock(void) {};
> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> index 9617b7df7c4d..97fe92c1f1d2 100644
> --- a/drivers/of/overlay.c
> +++ b/drivers/of/overlay.c
> @@ -974,8 +974,6 @@ static int of_overlay_apply(const void *fdt, struct device_node *tree,
>  		goto err_free_overlay_changeset;
>  	}
>  
> -	of_populate_phandle_cache();
> -
>  	ret = __of_changeset_apply_notify(&ovcs->cset);
>  	if (ret)
>  		pr_err("overlay apply changeset entry notify error %d\n", ret);
> @@ -1218,17 +1216,9 @@ int of_overlay_remove(int *ovcs_id)
>  
>  	list_del(&ovcs->ovcs_list);
>  
> -	/*
> -	 * Disable phandle cache.  Avoids race condition that would arise
> -	 * from removing cache entry when the associated node is deleted.
> -	 */
> -	of_free_phandle_cache();
> -
>  	ret_apply = 0;
>  	ret = __of_changeset_revert_entries(&ovcs->cset, &ret_apply);
>  
> -	of_populate_phandle_cache();
> -
>  	if (ret) {
>  		if (ret_apply)
>  			devicetree_state_flags |= DTSF_REVERT_FAIL;
>