Re: [PATCH V3] swiotlb: Split up single swiotlb lock

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jul 07, 2022 at 04:24:36AM -0400, Tianyu Lan wrote:
> From: Tianyu Lan <Tianyu.Lan@microsoft.com>
> 
> Traditionally swiotlb was not performance critical because it was only
> used for slow devices. But in some setups, like TDX/SEV confidential
> guests, all IO has to go through swiotlb. Currently swiotlb only has a
> single lock. Under high IO load with multiple CPUs this can lead to
> significat lock contention on the swiotlb lock.
> 
> This patch splits the swiotlb bounce buffer pool into individual areas
> which have their own lock. Each CPU tries to allocate in its own area
> first. Only if that fails does it search other areas. On freeing the
> allocation is freed into the area where the memory was originally
> allocated from.
> 
> Area number can be set via swiotlb kernel parameter and is default
> to be possible cpu number. If possible cpu number is not power of
> 2, area number will be round up to the next power of 2.
> 
> This idea from Andi Kleen patch(https://github.com/intel/tdx/commit/
> 4529b5784c141782c72ec9bd9a92df2b68cb7d45).

Thanks, this looks much better.  I think there is a small problem
with how default_nareas is set - we need to use 0 as the default
so that an explicit command line value of 1 works.  Als have you
checked the interaction with swiotlb_adjust_size in detail?

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 5536d2cd69d30..85b1c29dd0eb8 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -70,7 +70,7 @@ struct io_tlb_mem io_tlb_default_mem;
 phys_addr_t swiotlb_unencrypted_base;
 
 static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
-static unsigned long default_nareas = 1;
+static unsigned long default_nareas;
 
 /**
  * struct io_tlb_area - IO TLB memory area descriptor
@@ -90,7 +90,10 @@ struct io_tlb_area {
 
 static void swiotlb_adjust_nareas(unsigned int nareas)
 {
-	if (!is_power_of_2(nareas))
+	if (default_nareas)
+		return;
+
+	if (nareas > 1 && !is_power_of_2(nareas))
 		nareas = roundup_pow_of_two(nareas);
 
 	default_nareas = nareas;
@@ -338,8 +341,7 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
 		panic("%s: Failed to allocate %zu bytes align=0x%lx\n",
 		      __func__, alloc_size, PAGE_SIZE);
 
-	if (default_nareas == 1)
-		swiotlb_adjust_nareas(num_possible_cpus());
+	swiotlb_adjust_nareas(num_possible_cpus());
 
 	mem->areas = memblock_alloc(sizeof(struct io_tlb_area) *
 		default_nareas, SMP_CACHE_BYTES);
@@ -410,8 +412,7 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
 			(PAGE_SIZE << order) >> 20);
 	}
 
-	if (default_nareas == 1)
-		swiotlb_adjust_nareas(num_possible_cpus());
+	swiotlb_adjust_nareas(num_possible_cpus());
 
 	area_order = get_order(array_size(sizeof(*mem->areas),
 		default_nareas));