Re: [Patch v2 1/3] iommu: match the original algorithm

John Garry
2019-11-29T14:43:32Z

On 29/11/2019 00:48, Cong Wang wrote:
> The IOVA cache algorithm implemented in IOMMU code does not
> exactly match the original algorithm described in the paper.
>

which paper?

> 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.

I *quickly* tested this patch and got a small performance gain.

>
> Cc: Joerg Roedel <joro@8bytes.org>
> 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)) {

is this change strictly necessary?

> 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);

it is good to remove this from under the lock, apart from this change

> - 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--;

I'm not sure how appropriate the name "depot_size" is any longer.

> 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]);
> }
> }