Messages in this thread | | | Subject | Re: [PATCH] dma-debug: add a per-cpu cache to avoid lock contention | From | Robin Murphy <> | Date | Fri, 31 Jan 2020 13:14:36 +0000 |
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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.murphy@arm.com>
Thanks, Robin.
> Signed-off-by: Eric Dumazet <edumazet@google.com> > Cc: Christoph Hellwig <hch@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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