Re: [Patch v3 1/3] iommu: avoid unnecessary magazine allocations

[Robin Murphy <robin.murphy@arm.com>]

On 18/12/2019 4:39 am, Cong Wang wrote:
> The IOVA cache algorithm implemented in IOMMU code does not
> exactly match the original algorithm described in the paper
> "Magazines and Vmem: Extending the Slab Allocator to Many
> CPUs and Arbitrary Resources".
> 
> Particularly, it doesn't need to free the loaded empty magazine
> when trying to put it back to global depot. To make it work, we
> have to pre-allocate magazines in the depot and only recycle them
> when all of them are full.
> 
> Before this patch, rcache->depot[] contains either full or
> freed entries, after this patch, it contains either full or
> empty (but allocated) entries.

How much additional memory overhead does this impose (particularly on 
systems that may have many domains mostly used for large, long-term 
mappings)? I'm wary that trying to micro-optimise for the "churn network 
packets as fast as possible" case may penalise every other case, 
potentially quite badly. Lower-end embedded systems are using IOMMUs in 
front of their GPUs, video codecs, etc. precisely because they *don't* 
have much memory to spare (and thus need to scrape together large 
buffers out of whatever pages they can find).

But on the other hand, if we were to go down this route, then why is 
there any dynamic allocation/freeing left at all? Once both the depot 
and the rcaches are preallocated, then AFAICS it would make more sense 
to rework the overflow case in __iova_rcache_insert() to just free the 
IOVAs and swap the empty mag around rather than destroying and 
recreating it entirely.

Perhaps there's a reasonable compromise wherein we don't preallocate, 
but still 'free' empty magazines back to the depot, such that busy 
domains will quickly reach a steady-state. In fact, having now dug up 
the paper at this point of writing this reply, that appears to be what 
fig. 3.1b describes anyway - I don't see any mention of preallocating 
the depot.

Robin.

> 
> Together with a few other changes to make it exactly match
> the pseudo code in the paper.
> 
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: John Garry <john.garry@huawei.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---
>   drivers/iommu/iova.c | 45 +++++++++++++++++++++++++++-----------------
>   1 file changed, 28 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 41c605b0058f..cb473ddce4cf 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -862,12 +862,16 @@ static void init_iova_rcaches(struct iova_domain *iovad)
>   	struct iova_cpu_rcache *cpu_rcache;
>   	struct iova_rcache *rcache;
>   	unsigned int cpu;
> -	int i;
> +	int i, j;
>   
>   	for (i = 0; i < IOVA_RANGE_CACHE_MAX_SIZE; ++i) {
>   		rcache = &iovad->rcaches[i];
>   		spin_lock_init(&rcache->lock);
>   		rcache->depot_size = 0;
> +		for (j = 0; j < MAX_GLOBAL_MAGS; ++j) {
> +			rcache->depot[j] = iova_magazine_alloc(GFP_KERNEL);
> +			WARN_ON(!rcache->depot[j]);
> +		}
>   		rcache->cpu_rcaches = __alloc_percpu(sizeof(*cpu_rcache), cache_line_size());
>   		if (WARN_ON(!rcache->cpu_rcaches))
>   			continue;
> @@ -900,24 +904,30 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>   
>   	if (!iova_magazine_full(cpu_rcache->loaded)) {
>   		can_insert = true;
> -	} else if (!iova_magazine_full(cpu_rcache->prev)) {
> +	} else if (iova_magazine_empty(cpu_rcache->prev)) {
>   		swap(cpu_rcache->prev, cpu_rcache->loaded);
>   		can_insert = true;
>   	} else {
> -		struct iova_magazine *new_mag = iova_magazine_alloc(GFP_ATOMIC);
> +		spin_lock(&rcache->lock);
> +		if (rcache->depot_size < MAX_GLOBAL_MAGS) {
> +			swap(rcache->depot[rcache->depot_size], cpu_rcache->prev);
> +			swap(cpu_rcache->prev, cpu_rcache->loaded);
> +			rcache->depot_size++;
> +			can_insert = true;
> +		} else {
> +			mag_to_free = cpu_rcache->loaded;
> +		}
> +		spin_unlock(&rcache->lock);
> +
> +		if (mag_to_free) {
> +			struct iova_magazine *new_mag = iova_magazine_alloc(GFP_ATOMIC);
>   
> -		if (new_mag) {
> -			spin_lock(&rcache->lock);
> -			if (rcache->depot_size < MAX_GLOBAL_MAGS) {
> -				rcache->depot[rcache->depot_size++] =
> -						cpu_rcache->loaded;
> +			if (new_mag) {
> +				cpu_rcache->loaded = new_mag;
> +				can_insert = true;
>   			} else {
> -				mag_to_free = cpu_rcache->loaded;
> +				mag_to_free = NULL;
>   			}
> -			spin_unlock(&rcache->lock);
> -
> -			cpu_rcache->loaded = new_mag;
> -			can_insert = true;
>   		}
>   	}
>   
> @@ -963,14 +973,15 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
>   
>   	if (!iova_magazine_empty(cpu_rcache->loaded)) {
>   		has_pfn = true;
> -	} else if (!iova_magazine_empty(cpu_rcache->prev)) {
> +	} else if (iova_magazine_full(cpu_rcache->prev)) {
>   		swap(cpu_rcache->prev, cpu_rcache->loaded);
>   		has_pfn = true;
>   	} else {
>   		spin_lock(&rcache->lock);
>   		if (rcache->depot_size > 0) {
> -			iova_magazine_free(cpu_rcache->loaded);
> -			cpu_rcache->loaded = rcache->depot[--rcache->depot_size];
> +			swap(rcache->depot[rcache->depot_size - 1], cpu_rcache->prev);
> +			swap(cpu_rcache->prev, cpu_rcache->loaded);
> +			rcache->depot_size--;
>   			has_pfn = true;
>   		}
>   		spin_unlock(&rcache->lock);
> @@ -1019,7 +1030,7 @@ static void free_iova_rcaches(struct iova_domain *iovad)
>   			iova_magazine_free(cpu_rcache->prev);
>   		}
>   		free_percpu(rcache->cpu_rcaches);
> -		for (j = 0; j < rcache->depot_size; ++j)
> +		for (j = 0; j < MAX_GLOBAL_MAGS; ++j)
>   			iova_magazine_free(rcache->depot[j]);
>   	}
>   }
>