On 2020-01-30 8:14 pm, Eric Dumazet via iommu wrote:
Networking drivers very often have to replace one page with
another for their RX ring buffers.
A multi-queue NIC will severly hit a contention point
in dma-debug while grabbing free_entries_lock spinlock.
Adding a one entry per-cpu cache removes the need
to grab this spinlock twice per page replacement.
Tested on a 40Gbit mlx4 NIC, with 16 RX queues and about
1,000,000 replacements per second.
Looks like a decent benefit for minimal collateral impact - from what
I've seen, networking appears to be a particularly easy place to run
into awkward races, so mitigating even a little of the debug-induced
serialisation for the common case can only be good.
Reviewed-by: Robin Murphy <robin.mur...@arm.com>
Thanks,
Robin.
Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Christoph Hellwig <h...@lst.de>
---
kernel/dma/debug.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index
a310dbb1515e92c081f8f3f9a7290dd5e53fc889..b7221426ef49cf640db5bcb261b0817d714a3033
100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -97,6 +97,8 @@ static LIST_HEAD(free_entries);
/* Lock for the list above */
static DEFINE_SPINLOCK(free_entries_lock);
+static DEFINE_PER_CPU(struct dma_debug_entry *, dma_debug_entry_cache);
+
/* Global disable flag - will be set in case of an error */
static bool global_disable __read_mostly;
@@ -676,6 +678,10 @@ static struct dma_debug_entry *dma_entry_alloc(void)
struct dma_debug_entry *entry;
unsigned long flags;
+ entry = this_cpu_xchg(dma_debug_entry_cache, NULL);
+ if (entry)
+ goto end;
+
spin_lock_irqsave(&free_entries_lock, flags);
if (num_free_entries == 0) {
if (dma_debug_create_entries(GFP_ATOMIC)) {
@@ -690,7 +696,7 @@ static struct dma_debug_entry *dma_entry_alloc(void)
entry = __dma_entry_alloc();
spin_unlock_irqrestore(&free_entries_lock, flags);
-
+end:
#ifdef CONFIG_STACKTRACE
entry->stack_len = stack_trace_save(entry->stack_entries,
ARRAY_SIZE(entry->stack_entries),
@@ -705,6 +711,9 @@ static void dma_entry_free(struct dma_debug_entry *entry)
active_cacheline_remove(entry);
+ if (!this_cpu_cmpxchg(dma_debug_entry_cache, NULL, entry))
+ return;
+
/*
* add to beginning of the list - this way the entries are
* more likely cache hot when they are reallocated.
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