netdev - Serious performance degradation in Linux 4.15

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Date:   Fri, 9 Feb 2018 17:59:12 +0000
From:   Jon Maloy <jon.maloy@...csson.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...nel.org" <mingo@...nel.org>
CC:     "David Miller (davem@...emloft.net)" <davem@...emloft.net>
Subject: Serious performance degradation in Linux 4.15

The two commits 
d153b153446f7 (" sched/core: Fix wake_affine() performance regression") and
f2cdd9cc6c97 ("sched/core: Address more wake_affine() regressions")
are causing a serious performance degradation in Linux 4.5.

The effect is worst on TIPC, but even TCP is affected, as the figures below show. 


Command for TCP:
"netperf TCP_STREAM (netperf -n 4 -f m -c 4 -C 4 -P 1 -H 10.0.0.1 -t TCP_STREAM -l 10 -- -O THROUGHPUT)"

v4.15-rc1 without f2cdd9cc6c97e, d153b153446f7:   1293.67
V4.15 with the two commits:                                           1104.58 

i.e., a degradation of 17 % for TCP

Command for TIPC:
"netperf TIPC_STREAM (netperf -n 4 -f m -c 4 -C 4 -P 1 -H 10.0.0.1 -t TCP_STREAM -l 10 -- -O THROUGHPUT)"

v4.15-rc1 without f2cdd9cc6c97e, d153b153446f7:   786.22
V4.15 with the two commits:                                           223.18

i.e., a degradation of 71 % 

This is really bad, and I hope you have a plan for reintroducing this in some form.

BR
Jon Maloy







	v4.15-rc1->latest	diff (%)
"netperf TCP_STREAM (netperf -n 4 -f m -c 4 -C 4 -P 1 -H 10.0.0.1 -t TCP_STREAM -l 10 -- -O THROUGHPUT)"	1293.67	1104.58	-14.62%
"benchmark TIPC
(client_bench -c 1 -m 65000 -t)"	786.67	215.67	-72.58%
"netperf TIPC_STREAM
(netperf -n 4 -f m -c 4 -C 4 -P 1 -H 10.0.0.1 -t TIPC_STREAM -l 10 -- -O THROUGHPUT)"	786.22	223.18	-71.61%