Re: [PATCH 3/6] iova: Allow rcache range upper limit to be configurable

Robin Murphy
2021-03-19T16:26:41Z

On 2021-03-19 13:25, John Garry wrote:
> Some LLDs may request DMA mappings whose IOVA length exceeds that of the
> current rcache upper limit.
>
> This means that allocations for those IOVAs will never be cached, and
> always must be allocated and freed from the RB tree per DMA mapping cycle.
> This has a significant effect on performance, more so since commit
> 4e89dce72521 ("iommu/iova: Retry from last rb tree node if iova search
> fails"), as discussed at [0].
>
> Allow the range of cached IOVAs to be increased, by providing an API to set
> the upper limit, which is capped at IOVA_RANGE_CACHE_MAX_SIZE.
>
> For simplicity, the full range of IOVA rcaches is allocated and initialized
> at IOVA domain init time.
>
> Setting the range upper limit will fail if the domain is already live
> (before the tree contains IOVA nodes). This must be done to ensure any
> IOVAs cached comply with rule of size being a power-of-2.
>
> [0] https://lore.kernel.org/linux-iommu/20210129092120.1482-1-thunder.leizhen@huawei.com/
>
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
> drivers/iommu/iova.c | 37 +++++++++++++++++++++++++++++++++++--
> include/linux/iova.h | 11 ++++++++++-
> 2 files changed, 45 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index cecc74fb8663..d4f2f7fbbd84 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -49,6 +49,7 @@ init_iova_domain(struct iova_domain *iovad, unsigned long granule,
> iovad->flush_cb = NULL;
> iovad->fq = NULL;
> iovad->anchor.pfn_lo = iovad->anchor.pfn_hi = IOVA_ANCHOR;
> + iovad->rcache_max_size = IOVA_RANGE_CACHE_DEFAULT_SIZE;
> rb_link_node(&iovad->anchor.node, NULL, &iovad->rbroot.rb_node);
> rb_insert_color(&iovad->anchor.node, &iovad->rbroot);
> init_iova_rcaches(iovad);
> @@ -194,7 +195,7 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
> * rounding up anything cacheable to make sure that can't happen. The
> * order of the unadjusted size will still match upon freeing.
> */
> - if (fast && size < (1 << (IOVA_RANGE_CACHE_MAX_SIZE - 1)))
> + if (fast && size < (1 << (iovad->rcache_max_size - 1)))
> size = roundup_pow_of_two(size);
>
> if (size_aligned)
> @@ -901,7 +902,7 @@ static bool iova_rcache_insert(struct iova_domain *iovad, unsigned long pfn,
> {
> unsigned int log_size = order_base_2(size);
>
> - if (log_size >= IOVA_RANGE_CACHE_MAX_SIZE)
> + if (log_size >= iovad->rcache_max_size)
> return false;
>
> return __iova_rcache_insert(iovad, &iovad->rcaches[log_size], pfn);
> @@ -946,6 +947,38 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
> return iova_pfn;
> }
>
> +void iova_rcache_set_upper_limit(struct iova_domain *iovad,
> + unsigned long iova_len)
> +{
> + unsigned int rcache_index = order_base_2(iova_len) + 1;
> + struct rb_node *rb_node = &iovad->anchor.node;
> + unsigned long flags;
> + int count = 0;
> +
> + spin_lock_irqsave(&iovad->iova_rbtree_lock, flags);
> + if (rcache_index <= iovad->rcache_max_size)
> + goto out;
> +
> + while (1) {
> + rb_node = rb_prev(rb_node);
> + if (!rb_node)
> + break;
> + count++;
> + }
> +
> + /*
> + * If there are already IOVA nodes present in the tree, then don't
> + * allow range upper limit to be set.
> + */
> + if (count != iovad->reserved_node_count)
> + goto out;
> +
> + iovad->rcache_max_size = min_t(unsigned long, rcache_index,
> + IOVA_RANGE_CACHE_MAX_SIZE);
> +out:
> + spin_unlock_irqrestore(&iovad->iova_rbtree_lock, flags);
> +}
> +
> /*
> * Try to satisfy IOVA allocation range from rcache. Fail if requested
> * size is too big or the DMA limit we are given isn't satisfied by the
> diff --git a/include/linux/iova.h b/include/linux/iova.h
> index fd3217a605b2..952b81b54ef7 100644
> --- a/include/linux/iova.h
> +++ b/include/linux/iova.h
> @@ -25,7 +25,8 @@ struct iova {
> struct iova_magazine;
> struct iova_cpu_rcache;
>
> -#define IOVA_RANGE_CACHE_MAX_SIZE 6 /* log of max cached IOVA range size (in pages) */
> +#define IOVA_RANGE_CACHE_DEFAULT_SIZE 6
> +#define IOVA_RANGE_CACHE_MAX_SIZE 10 /* log of max cached IOVA range size (in pages) */

No.

And why? If we're going to allocate massive caches anyway, whatever is
the point of *not* using all of them?

It only makes sense for a configuration knob to affect the actual rcache
and depot allocations - that way, big high-throughput systems with
plenty of memory can spend it on better performance, while small systems
- that often need IOMMU scatter-gather precisely *because* memory is
tight and thus easily fragmented - don't have to pay the (not
insignificant) cost for caches they don't need.

Robin.

> #define MAX_GLOBAL_MAGS 32 /* magazines per bin */
>
> struct iova_rcache {
> @@ -74,6 +75,7 @@ struct iova_domain {
> unsigned long start_pfn; /* Lower limit for this domain */
> unsigned long dma_32bit_pfn;
> unsigned long max32_alloc_size; /* Size of last failed allocation */
> + unsigned long rcache_max_size; /* Upper limit of cached IOVA RANGE */
> struct iova_fq __percpu *fq; /* Flush Queue */
>
> atomic64_t fq_flush_start_cnt; /* Number of TLB flushes that
> @@ -158,6 +160,8 @@ int init_iova_flush_queue(struct iova_domain *iovad,
> struct iova *find_iova(struct iova_domain *iovad, unsigned long pfn);
> void put_iova_domain(struct iova_domain *iovad);
> void free_cpu_cached_iovas(unsigned int cpu, struct iova_domain *iovad);
> +void iova_rcache_set_upper_limit(struct iova_domain *iovad,
> + unsigned long iova_len);
> #else
> static inline int iova_cache_get(void)
> {
> @@ -238,6 +242,11 @@ static inline void free_cpu_cached_iovas(unsigned int cpu,
> struct iova_domain *iovad)
> {
> }
> +
> +static inline void iova_rcache_set_upper_limit(struct iova_domain *iovad,
> + unsigned long iova_len)
> +{
> +}
> #endif
>
> #endif
>