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

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Date: Thu, 1 Feb 2024 14:28:58 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: robin.murphy@....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 Thu, Feb 1, 2024 at 11:30 AM Pasha Tatashin
<pasha.tatashin@...een.com> 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.
>
> 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);

Could this slab cache be merged with a compatible one in the slab
code? If this happens, do we still get a separate entry in
/proc/slabinfo?

It may be useful to use SLAB_NO_MERGE if the purpose is to
specifically have observability into this slab cache, but the comments
above the flag make me think I may be misunderstanding it.