When __inet_hash_connect() has to try many 4-tuples before
finding an available one, we see a high spinlock cost from
the many spin_lock_bh(&head->lock) performed in its loop.
This patch adds an RCU lookup to avoid the spinlock cost.
check_established() gets a new @rcu_lookup argument.
First reason is to not make any changes while head->lock
is not held.
Second reason is to not make this RCU lookup a second time
after the spinlock has been acquired.
Tested:
Server:
ulimit -n 40000; neper/tcp_crr -T 200 -F 30000 -6 --nolog
Client:
ulimit -n 40000; neper/tcp_crr -T 200 -F 30000 -6 --nolog -c -H server
Before series:
utime_start=0.288582
utime_end=1.548707
stime_start=20.637138
stime_end=2002.489845
num_transactions=484453
latency_min=0.156279245
latency_max=20.922042756
latency_mean=1.546521274
latency_stddev=3.936005194
num_samples=312537
throughput=47426.00
perf top on the client:
49.54% [kernel] [k] _raw_spin_lock
25.87% [kernel] [k] _raw_spin_lock_bh
5.97% [kernel] [k] queued_spin_lock_slowpath
5.67% [kernel] [k] __inet_hash_connect
3.53% [kernel] [k] __inet6_check_established
3.48% [kernel] [k] inet6_ehashfn
0.64% [kernel] [k] rcu_all_qs
After this series:
utime_start=0.271607
utime_end=3.847111
stime_start=18.407684
stime_end=1997.485557
num_transactions=1350742
latency_min=0.014131929
latency_max=17.895073144
latency_mean=0.505675853 # Nice reduction of latency metrics
latency_stddev=2.125164772
num_samples=307884
throughput=139866.80 # 190 % increase
perf top on client:
56.86% [kernel] [k] __inet6_check_established
17.96% [kernel] [k] __inet_hash_connect
13.88% [kernel] [k] inet6_ehashfn
2.52% [kernel] [k] rcu_all_qs
2.01% [kernel] [k] __cond_resched
0.41% [kernel] [k] _raw_spin_lock
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Tested-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250302124237.3913746-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
86c2bc293b