[PATCH v3 3/4] iommu/iova: Flush CPU rcache for when a depot fills

[John Garry]

Leizhen reported some time ago that IOVA performance may degrade over time
[0], but unfortunately his solution to fix this problem was not given
attention.

To summarize, the issue is that as time goes by, the CPU rcache and depot
rcache continue to grow. As such, IOVA RB tree access time also continues
to grow.

At a certain point, a depot may become full, and also some CPU rcaches may
also be full when inserting another IOVA is attempted. For this scenario,
currently the "loaded" CPU rcache is freed and a new one is created. This
freeing means that many IOVAs in the RB tree need to be freed, which
makes IO throughput performance fall off a cliff in some storage scenarios:

Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6314MB/0KB/0KB /s] [1616K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [5669MB/0KB/0KB /s] [1451K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6031MB/0KB/0KB /s] [1544K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6673MB/0KB/0KB /s] [1708K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6705MB/0KB/0KB /s] [1717K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6031MB/0KB/0KB /s] [1544K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6761MB/0KB/0KB /s] [1731K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6705MB/0KB/0KB /s] [1717K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6685MB/0KB/0KB /s] [1711K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6178MB/0KB/0KB /s] [1582K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [6731MB/0KB/0KB /s] [1723K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [2387MB/0KB/0KB /s] [611K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [2689MB/0KB/0KB /s] [688K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [2278MB/0KB/0KB /s] [583K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [1288MB/0KB/0KB /s] [330K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [1632MB/0KB/0KB /s] [418K/0/0 iops]
Jobs: 12 (f=12): [RRRRRRRRRRRR] [0.0% done] [1765MB/0KB/0KB /s] [452K/0/0 iops]

And continue in this fashion, without recovering. Note that in this
example it was required to wait 16 hours for this to occur. Also note that
IO throughput also becomes gradually becomes more unstable leading up to
this point.

This problem is only seen for non-strict mode. For strict mode, the rcaches
stay quite compact.

As a solution to this issue, judge that the IOVA caches have grown too big
when cached magazines need to be free, and just flush all the CPUs rcaches
instead.

The depot rcaches, however, are not flushed, as they can be used to
immediately replenish active CPUs.

In future, some IOVA compaction could be implemented to solve the
instabilty issue, which I figure could be quite complex to implement.

[0] https://lore.kernel.org/linux-iommu/20190815121104.29140-3-thunder.leizhen@xxxxxxxxxx/

Analyzed-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
Reported-by: Xiang Chen <chenxiang66@xxxxxxxxxxxxx>
Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
---
 drivers/iommu/iova.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 1f3f0f8b12e0..386005055aca 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -901,7 +901,6 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
 				 struct iova_rcache *rcache,
 				 unsigned long iova_pfn)
 {
-	struct iova_magazine *mag_to_free = NULL;
 	struct iova_cpu_rcache *cpu_rcache;
 	bool can_insert = false;
 	unsigned long flags;
@@ -923,13 +922,12 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
 				if (cpu_rcache->loaded)
 					rcache->depot[rcache->depot_size++] =
 							cpu_rcache->loaded;
-			} else {
-				mag_to_free = cpu_rcache->loaded;
+				can_insert = true;
+				cpu_rcache->loaded = new_mag;
 			}
 			spin_unlock(&rcache->lock);
-
-			cpu_rcache->loaded = new_mag;
-			can_insert = true;
+			if (!can_insert)
+				iova_magazine_free(new_mag);
 		}
 	}
 
@@ -938,10 +936,8 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
 
 	spin_unlock_irqrestore(&cpu_rcache->lock, flags);
 
-	if (mag_to_free) {
-		iova_magazine_free_pfns(mag_to_free, iovad);
-		iova_magazine_free(mag_to_free);
-	}
+	if (!can_insert)
+		free_all_cpu_cached_iovas(iovad);
 
 	return can_insert;
 }
-- 
2.26.2