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Date: Mon, 5 Feb 2024 15:31:21 +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, yosryahmed@...gle.com
Subject: Re: [PATCH v2] iommu/iova: use named kmem_cache for iova magazines
On 2024-02-02 7:28 pm, Pasha Tatashin wrote:
> The magazine buffers can take gigabytes of kmem memory, dominating all
> other allocations. For observability purpose 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.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
> drivers/iommu/iova.c | 70 ++++++++++++++++++++++++++------------------
> 1 file changed, 42 insertions(+), 28 deletions(-)
>
> Changelog:
> v2: - Use iova_cache_get/iova_cache_put to allocate/free
> "iova_magazine_cache" as suggested by Robin Murphy
> - Minor fix in the commit log.
>
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index d30e453d0fb4..88255f9443b5 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -237,6 +237,35 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
> return -ENOMEM;
> }
>
> +/*
> + * Magazine caches for IOVA ranges. For an introduction to magazines,
> + * see the USENIX 2001 paper "Magazines and Vmem: Extending the Slab
> + * Allocator to Many CPUs and Arbitrary Resources" by Bonwick and Adams.
> + * For simplicity, we use a static magazine size and don't implement the
> + * dynamic size tuning described in the paper.
> + */
> +
> +/*
> + * As kmalloc's buffer size is fixed to power of 2, 127 is chosen to
> + * assure size of 'iova_magazine' to be 1024 bytes, so that no memory
> + * will be wasted. Since only full magazines are inserted into the depot,
> + * we don't need to waste PFN capacity on a separate list head either.
> + */
> +#define IOVA_MAG_SIZE 127
> +
> +#define IOVA_DEPOT_DELAY msecs_to_jiffies(100)
> +
> +struct iova_magazine {
> + union {
> + unsigned long size;
> + struct iova_magazine *next;
> + };
> + unsigned long pfns[IOVA_MAG_SIZE];
> +};
> +
> +static_assert(!(sizeof(struct iova_magazine) & (sizeof(struct iova_magazine) - 1)));
Hmm, moving all this lot is a bit nasty, but I can see the current code
layout forces you to need sizeof(struct iova_magazine) earlier... :(
> +
> +static struct kmem_cache *iova_magazine_cache;
> static struct kmem_cache *iova_cache;
> static unsigned int iova_cache_users;
> static DEFINE_MUTEX(iova_cache_mutex);
> @@ -275,6 +304,16 @@ int iova_cache_get(void)
> pr_err("Couldn't create iova cache\n");
> return -ENOMEM;
> }
> +
> + iova_magazine_cache = kmem_cache_create("iommu_iova_magazine",
> + sizeof(struct iova_magazine),
> + 0, SLAB_HWCACHE_ALIGN, NULL);
> + if (!iova_magazine_cache) {
> + cpuhp_remove_multi_state(CPUHP_IOMMU_IOVA_DEAD);
> + mutex_unlock(&iova_cache_mutex);
> + pr_err("Couldn't create iova magazine cache\n");
> + return -ENOMEM;
And you're also strictly missing cleanup of iova_cache here. However
again I think that's more the fault of the existing code for being
clunky. Rather than drag out this review any more, though, I figured I'd
just sort it all out - patches incoming :)
Cheers,
Robin.
> + }
> }
>
> iova_cache_users++;
> @@ -295,6 +334,7 @@ void iova_cache_put(void)
> if (!iova_cache_users) {
> cpuhp_remove_multi_state(CPUHP_IOMMU_IOVA_DEAD);
> kmem_cache_destroy(iova_cache);
> + kmem_cache_destroy(iova_magazine_cache);
> }
> mutex_unlock(&iova_cache_mutex);
> }
> @@ -612,32 +652,6 @@ reserve_iova(struct iova_domain *iovad,
> }
> EXPORT_SYMBOL_GPL(reserve_iova);
>
> -/*
> - * Magazine caches for IOVA ranges. For an introduction to magazines,
> - * see the USENIX 2001 paper "Magazines and Vmem: Extending the Slab
> - * Allocator to Many CPUs and Arbitrary Resources" by Bonwick and Adams.
> - * For simplicity, we use a static magazine size and don't implement the
> - * dynamic size tuning described in the paper.
> - */
> -
> -/*
> - * As kmalloc's buffer size is fixed to power of 2, 127 is chosen to
> - * assure size of 'iova_magazine' to be 1024 bytes, so that no memory
> - * will be wasted. Since only full magazines are inserted into the depot,
> - * we don't need to waste PFN capacity on a separate list head either.
> - */
> -#define IOVA_MAG_SIZE 127
> -
> -#define IOVA_DEPOT_DELAY msecs_to_jiffies(100)
> -
> -struct iova_magazine {
> - union {
> - unsigned long size;
> - struct iova_magazine *next;
> - };
> - unsigned long pfns[IOVA_MAG_SIZE];
> -};
> -static_assert(!(sizeof(struct iova_magazine) & (sizeof(struct iova_magazine) - 1)));
>
> struct iova_cpu_rcache {
> spinlock_t lock;
> @@ -658,7 +672,7 @@ 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 +681,7 @@ 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
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