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Date: Fri, 2 Sep 2022 18:12:43 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <kuni1840@...il.com>,
<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v3 net-next 3/5] tcp: Access &tcp_hashinfo via net.
From: Eric Dumazet <edumazet@...gle.com>
Date: Fri, 2 Sep 2022 17:53:18 -0700
> On Fri, Sep 2, 2022 at 5:44 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > From: Kuniyuki Iwashima <kuniyu@...zon.com>
> > Date: Thu, 1 Sep 2022 15:12:16 -0700
> > > From: Eric Dumazet <edumazet@...gle.com>
> > > Date: Thu, 1 Sep 2022 14:30:43 -0700
> > > > On Thu, Sep 1, 2022 at 2:25 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > > > >
> > > > > From: Paolo Abeni <pabeni@...hat.com>
> > > >
> > > > > > /Me is thinking aloud...
> > > > > >
> > > > > > I'm wondering if the above has some measurable negative effect for
> > > > > > large deployments using only the main netns?
> > > > > >
> > > > > > Specifically, are net->ipv4.tcp_death_row and net->ipv4.tcp_death_row-
> > > > > > >hashinfo already into the working set data for established socket?
> > > > > > Would the above increase the WSS by 2 cache-lines?
> > > > >
> > > > > Currently, the death_row and hashinfo are touched around tw sockets or
> > > > > connect(). If connections on the deployment are short-lived or frequently
> > > > > initiated by itself, that would be host and included in WSS.
> > > > >
> > > > > If the workload is server and there's no active-close() socket or
> > > > > connections are long-lived, then it might not be included in WSS.
> > > > > But I think it's not likely than the former if the deployment is
> > > > > large enough.
> > > > >
> > > > > If this change had large impact, then we could revert fbb8295248e1
> > > > > which converted net->ipv4.tcp_death_row into pointer for 0dad4087a86a
> > > > > that tried to fire a TW timer after netns is freed, but 0dad4087a86a
> > > > > has already reverted.
> > > >
> > > >
> > > > Concern was fast path.
> > > >
> > > > Each incoming packet does a socket lookup.
> > > >
> > > > Fetching hashinfo (instead of &tcp_hashinfo) with a dereference of a
> > > > field in 'struct net' might inccurr a new cache line miss.
> > > >
> > > > Previously, first cache line of tcp_info was enough to bring a lot of
> > > > fields in cpu cache.
> > >
> > > Ok, let me test on that if there could be regressions.
> >
> > I tested tcp_hashinfo vs tcp_death_row->hashinfo with super_netperf
> > and collected HW cache-related metrics with perf.
> >
> > After the patch the number of L1 miss seems to increase, but the
> > instructions per cycle also increases, and cache miss rate did not
> > change. Also, there was not performance regression for netperf.
> >
> >
> > Tested:
> >
> > # cat perf_super_netperf
> > echo 0 > /proc/sys/kernel/nmi_watchdog
> > echo 3 > /proc/sys/vm/drop_caches
> >
> > perf stat -a \
> > -e cycles,instructions,cache-references,cache-misses,bus-cycles \
> > -e L1-dcache-loads,L1-dcache-load-misses,L1-dcache-stores \
> > -e dTLB-loads,dTLB-load-misses \
> > -e LLC-loads,LLC-load-misses,LLC-stores \
> > ./super_netperf $(($(nproc) * 2)) -H 10.0.0.142 -l 60 -fM
> >
> > echo 1 > /proc/sys/kernel/nmi_watchdog
> >
> >
> > Before:
> >
> > # ./perf_super_netperf
> > 2929.81
> >
> > Performance counter stats for 'system wide':
> >
> > 494,002,600,338 cycles (23.07%)
> > 241,230,662,890 instructions # 0.49 insn per cycle (30.76%)
> > 6,303,603,008 cache-references (38.45%)
> > 1,421,440,332 cache-misses # 22.550 % of all cache refs (46.15%)
> > 4,861,179,308 bus-cycles (46.15%)
> > 65,410,735,599 L1-dcache-loads (46.15%)
> > 12,647,247,339 L1-dcache-load-misses # 19.34% of all L1-dcache accesses (30.77%)
> > 32,912,656,369 L1-dcache-stores (30.77%)
> > 66,015,779,361 dTLB-loads (30.77%)
> > 81,293,994 dTLB-load-misses # 0.12% of all dTLB cache accesses (30.77%)
> > 2,946,386,949 LLC-loads (30.77%)
> > 257,223,942 LLC-load-misses # 8.73% of all LL-cache accesses (30.77%)
> > 1,183,820,461 LLC-stores (15.38%)
> >
> > 62.132250590 seconds time elapsed
> >
>
> This test will not be able to see a difference really...
>
> What is needed is to measure the latency when nothing at all is in the caches.
>
> Vast majority of real world TCP traffic is light or moderate.
> Packets are received and cpu has to bring X cache lines into L1 in
> order to process one packet.
>
> We slowly are increasing X over time :/
>
> pahole is your friend, more than a stress-test.
Here's pahole result on my local build. As Paolo said, we
need 2 cachelines for tcp_death_row and the hashinfo?
How about moving hashinfo as the first member of struct
inet_timewait_death_row and convert it to just struct
instead of pointer so that we need 1 cache line to read
hashinfo?
$ pahole -C netns_ipv4,inet_timewait_death_row vmlinux
struct netns_ipv4 {
struct inet_timewait_death_row * tcp_death_row; /* 0 8 */
struct ctl_table_header * forw_hdr; /* 8 8 */
struct ctl_table_header * frags_hdr; /* 16 8 */
struct ctl_table_header * ipv4_hdr; /* 24 8 */
struct ctl_table_header * route_hdr; /* 32 8 */
struct ctl_table_header * xfrm4_hdr; /* 40 8 */
struct ipv4_devconf * devconf_all; /* 48 8 */
struct ipv4_devconf * devconf_dflt; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
...
};
struct inet_timewait_death_row {
refcount_t tw_refcount; /* 0 4 */
/* XXX 60 bytes hole, try to pack */
/* --- cacheline 1 boundary (64 bytes) --- */
struct inet_hashinfo * hashinfo __attribute__((__aligned__(64))); /* 64 8 */
int sysctl_max_tw_buckets; /* 72 4 */
/* size: 128, cachelines: 2, members: 3 */
/* sum members: 16, holes: 1, sum holes: 60 */
/* padding: 52 */
/* forced alignments: 1, forced holes: 1, sum forced holes: 60 */
} __attribute__((__aligned__(64)));
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