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While using a MQ + NETEM setup, I had confirmation that the default
timer migration ( /proc/sys/kernel/timer_migration ) is killing us.
Installing this on a receiver side of a TCP_STREAM test, (NIC has 8 TX
queues) :
EST="est 1sec 4sec"
for ETH in eth1
do
tc qd del dev $ETH root 2>/dev/null
tc qd add dev $ETH root handle 1: mq
tc qd add dev $ETH parent 1:1 $EST netem limit 70000 delay 6ms
tc qd add dev $ETH parent 1:2 $EST netem limit 70000 delay 8ms
tc qd add dev $ETH parent 1:3 $EST netem limit 70000 delay 10ms
tc qd add dev $ETH parent 1:4 $EST netem limit 70000 delay 12ms
tc qd add dev $ETH parent 1:5 $EST netem limit 70000 delay 14ms
tc qd add dev $ETH parent 1:6 $EST netem limit 70000 delay 16ms
tc qd add dev $ETH parent 1:7 $EST netem limit 80000 delay 18ms
tc qd add dev $ETH parent 1:8 $EST netem limit 90000 delay 20ms
done
We can see that timers get migrated into a single cpu, presumably idle
at the time timers are set up.
Then all qdisc dequeues run from this cpu and huge lock contention
happens. This single cpu is stuck in softirq mode and cannot dequeue
fast enough.
39.24% [kernel] [k] _raw_spin_lock
2.65% [kernel] [k] netem_enqueue
1.80% [kernel] [k] netem_dequeue
1.63% [kernel] [k] copy_user_enhanced_fast_string
1.45% [kernel] [k] _raw_spin_lock_bh
By pinning qdisc timers on the cpu running the qdisc, we respect proper
XPS setting and remove this lock contention.
5.84% [kernel] [k] netem_enqueue
4.83% [kernel] [k] _raw_spin_lock
2.92% [kernel] [k] copy_user_enhanced_fast_string
Current Qdiscs that benefit from this change are :
netem, cbq, fq, hfsc, tbf, htb.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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The send_check logic was only interesting in cases of TCP offload and
UDP UFO where the checksum needed to be initialized to the pseudo
header checksum. Now we've moved that logic into the related
gso_segment functions so gso_send_check is no longer needed.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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In udp[46]_ufo_send_check the UDP checksum initialized to the pseudo
header checksum. We can move this logic into udp[46]_ufo_fragment.
After this change udp[64]_ufo_send_check is a no-op.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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In tcp_v[46]_gso_send_check the TCP checksum is initialized to the
pseudo header checksum using __tcp_v[46]_send_check. We can move this
logic into new tcp[46]_gso_segment functions to be done when
ip_summed != CHECKSUM_PARTIAL (ip_summed == CHECKSUM_PARTIAL should be
the common case, possibly always true when taking GSO path). After this
change tcp_v[46]_gso_send_check is no-op.
Signed-off-by: Tom Herbert <therbert@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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