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Date: Fri, 2 Sep 2022 19:50:34 -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 19:30:31 -0700
> On Fri, Sep 2, 2022 at 6:44 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > From: Kuniyuki Iwashima <kuniyu@...zon.com>
> > Date: Fri, 2 Sep 2022 18:12:43 -0700
> > > 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?
> >
> > Like this.
> >
> > $ pahole -EC netns_ipv4 vmlinux
> > struct netns_ipv4 {
> > struct inet_timewait_death_row {
> > struct inet_hashinfo * hashinfo __attribute__((__aligned__(64))); /* 0 8 */
> > /* typedef refcount_t */ struct refcount_struct {
> > /* typedef atomic_t */ struct {
> > int counter; /* 8 4 */
> > } refs; /* 8 4 */
> > } tw_refcount; /* 8 4 */
> > int sysctl_max_tw_buckets; /* 12 4 */
> > } tcp_death_row __attribute__((__aligned__(64))) __attribute__((__aligned__(64))); /* 0 64 */
> >
> > /* XXX last struct has 48 bytes of padding */
> >
> > /* --- cacheline 1 boundary (64 bytes) --- */
> > ...
> > } __attribute__((__aligned__(64)));
> >
> >
> > ---8<---
> > diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
> > index 6320a76cefdc..dee53193d258 100644
> > --- a/include/net/netns/ipv4.h
> > +++ b/include/net/netns/ipv4.h
> > @@ -32,16 +32,15 @@ struct ping_group_range {
> > struct inet_hashinfo;
> >
> > struct inet_timewait_death_row {
> > - refcount_t tw_refcount;
> > -
> > struct inet_hashinfo *hashinfo ____cacheline_aligned_in_smp;
> > + refcount_t tw_refcount;
>
> This would be very bad. tw_refcount would share a cache line with hashinfo.
>
> false sharing is more problematic than a cache line miss in read mode.
Ah, exactly.
Then I will add ____cacheline_aligned_in_smp to tw_refcount, instead of
keeping the original order, to avoid invalidation by sysctl_max_tw_buckets
change, which wouldn't be so frequently done though.
struct inet_timewait_death_row {
- refcount_t tw_refcount;
-
- struct inet_hashinfo *hashinfo ____cacheline_aligned_in_smp;
+ struct inet_hashinfo *hashinfo;
+ refcount_t tw_refcount ____cacheline_aligned_in_smp;
int sysctl_max_tw_buckets;
};
>
> > int sysctl_max_tw_buckets;
> > };
> >
> > struct tcp_fastopen_context;
> >
> > struct netns_ipv4 {
> > - struct inet_timewait_death_row *tcp_death_row;
> > + struct inet_timewait_death_row tcp_death_row;
> >
> > #ifdef CONFIG_SYSCTL
> > struct ctl_table_header *forw_hdr;
> > ---8<---
> >
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