linux-kernel - Re: [PATCH] iommu/iova: use named kmem

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Date: Thu, 1 Feb 2024 20:56:04 +0000
From: Robin Murphy <robin.murphy@....com>
To: Pasha Tatashin <pasha.tatashin@...een.com>, joro@...tes.org,
 will@...nel.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, rientjes@...gle.com
Subject: Re: [PATCH] iommu/iova: use named kmem_cache for iova magazines

On 2024-02-01 7:30 pm, Pasha Tatashin wrote:
> From: Pasha Tatashin <pasha.tatashin@...een.com>
> 
> The magazine buffers can take gigabytes of kmem memory, dominating all
> other allocations. For observability prurpose create named slab cache so
> the iova magazine memory overhead can be clearly observed.
> 
> With this change:
> 
>> slabtop -o | head
>   Active / Total Objects (% used)    : 869731 / 952904 (91.3%)
>   Active / Total Slabs (% used)      : 103411 / 103974 (99.5%)
>   Active / Total Caches (% used)     : 135 / 211 (64.0%)
>   Active / Total Size (% used)       : 395389.68K / 411430.20K (96.1%)
>   Minimum / Average / Maximum Object : 0.02K / 0.43K / 8.00K
> 
> OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME
> 244412 244239 99%    1.00K  61103       4    244412K iommu_iova_magazine
>   91636  88343 96%    0.03K    739     124      2956K kmalloc-32
>   75744  74844 98%    0.12K   2367      32      9468K kernfs_node_cache
> 
> On this machine it is now clear that magazine use 242M of kmem memory.

Hmm, something smells there...

In the "worst" case there should be a maximum of 6 * 2 * 
num_online_cpus() empty magazines in the iova_cpu_rcache structures, 
i.e., 12KB per CPU. Under normal use those will contain at least some 
PFNs, but mainly every additional magazine stored in a depot is full 
with 127 PFNs, and each one of those PFNs is backed by a 40-byte struct 
iova, i.e. ~5KB per 1KB magazine. Unless that machine has many thousands 
of CPUs, if iova_magazine allocations are the top consumer of memory 
then something's gone wrong.

Thanks,
Robin.

> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>   drivers/iommu/iova.c | 57 +++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 54 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index d30e453d0fb4..617bbc2b79f5 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -630,6 +630,10 @@ EXPORT_SYMBOL_GPL(reserve_iova);
>   
>   #define IOVA_DEPOT_DELAY msecs_to_jiffies(100)
>   
> +static struct kmem_cache *iova_magazine_cache;
> +static unsigned int iova_magazine_cache_users;
> +static DEFINE_MUTEX(iova_magazine_cache_mutex);
> +
>   struct iova_magazine {
>   	union {
>   		unsigned long size;
> @@ -654,11 +658,51 @@ struct iova_rcache {
>   	struct delayed_work work;
>   };
>   
> +static int iova_magazine_cache_init(void)
> +{
> +	int ret = 0;
> +
> +	mutex_lock(&iova_magazine_cache_mutex);
> +
> +	iova_magazine_cache_users++;
> +	if (iova_magazine_cache_users > 1)
> +		goto out_unlock;
> +
> +	iova_magazine_cache = kmem_cache_create("iommu_iova_magazine",
> +						sizeof(struct iova_magazine),
> +						0, SLAB_HWCACHE_ALIGN, NULL);
> +
> +	if (!iova_magazine_cache) {
> +		pr_err("Couldn't create iova magazine cache\n");
> +		ret = -ENOMEM;
> +	}
> +
> +out_unlock:
> +	mutex_unlock(&iova_magazine_cache_mutex);
> +
> +	return ret;
> +}
> +
> +static void iova_magazine_cache_fini(void)
> +{
> +	mutex_lock(&iova_magazine_cache_mutex);
> +
> +	if (WARN_ON(!iova_magazine_cache_users))
> +		goto out_unlock;
> +
> +	iova_magazine_cache_users--;
> +	if (!iova_magazine_cache_users)
> +		kmem_cache_destroy(iova_magazine_cache);
> +
> +out_unlock:
> +	mutex_unlock(&iova_magazine_cache_mutex);
> +}
> +
>   static struct iova_magazine *iova_magazine_alloc(gfp_t flags)
>   {
>   	struct iova_magazine *mag;
>   
> -	mag = kmalloc(sizeof(*mag), flags);
> +	mag = kmem_cache_alloc(iova_magazine_cache, flags);
>   	if (mag)
>   		mag->size = 0;
>   
> @@ -667,7 +711,7 @@ static struct iova_magazine *iova_magazine_alloc(gfp_t flags)
>   
>   static void iova_magazine_free(struct iova_magazine *mag)
>   {
> -	kfree(mag);
> +	kmem_cache_free(iova_magazine_cache, mag);
>   }
>   
>   static void
> @@ -766,11 +810,17 @@ int iova_domain_init_rcaches(struct iova_domain *iovad)
>   	unsigned int cpu;
>   	int i, ret;
>   
> +	ret = iova_magazine_cache_init();
> +	if (ret)
> +		return -ENOMEM;
> +
>   	iovad->rcaches = kcalloc(IOVA_RANGE_CACHE_MAX_SIZE,
>   				 sizeof(struct iova_rcache),
>   				 GFP_KERNEL);
> -	if (!iovad->rcaches)
> +	if (!iovad->rcaches) {
> +		iova_magazine_cache_fini();
>   		return -ENOMEM;
> +	}
>   
>   	for (i = 0; i < IOVA_RANGE_CACHE_MAX_SIZE; ++i) {
>   		struct iova_cpu_rcache *cpu_rcache;
> @@ -948,6 +998,7 @@ static void free_iova_rcaches(struct iova_domain *iovad)
>   
>   	kfree(iovad->rcaches);
>   	iovad->rcaches = NULL;
> +	iova_magazine_cache_fini();
>   }
>   
>   /*