Re: [RFC PATCH v3 2/7] proc: Reduce cache miss in {snmp,netstat}_seq_show

From: hejianet
Date: Thu Sep 22 2016 - 01:39:16 EST

On 9/22/16 2:24 AM, Marcelo wrote:
On Thu, Sep 22, 2016 at 12:18:46AM +0800, hejianet wrote:
Hi Marcelo

sorry for the late, just came back from a vacation.
Hi, no problem. Hope your batteries are recharged now :-)

On 9/14/16 7:55 PM, Marcelo wrote:
Hi Jia,

On Wed, Sep 14, 2016 at 01:58:42PM +0800, hejianet wrote:
Hi Marcelo

On 9/13/16 2:57 AM, Marcelo wrote:
On Fri, Sep 09, 2016 at 02:33:57PM +0800, Jia He wrote:
This is to use the generic interface snmp_get_cpu_field{,64}_batch to
aggregate the data by going through all the items of each cpu sequentially.
Then snmp_seq_show and netstat_seq_show are split into 2 parts to avoid build
warning "the frame size" larger than 1024 on s390.
Yeah about that, did you test it with stack overflow detection?
These arrays can be quite large.

One more below..
Do you think it is acceptable if the stack usage is a little larger than 1024?
e.g. 1120
I can't find any other way to reduce the stack usage except use "static" before
unsigned long buff[TCP_MIB_MAX]

PS. sizeof buff is about TCP_MIB_MAX(116)*8=928
That's pretty much the question. Linux has the option on some archs to
run with 4Kb (4KSTACKS option), so this function alone would be using
25% of it in this last case. While on x86_64, it uses 16Kb (6538b8ea886e
("x86_64: expand kernel stack to 16K")).

Adding static to it is not an option as it actually makes the variable
shared amongst the CPUs (and then you have concurrency issues), plus the
fact that it's always allocated, even while not in use.

Others here certainly know better than me if it's okay to make such
usage of the stach.
What about this patch instead?
It is a trade-off. I split the aggregation process into 2 parts, it will
increase the cache miss a little bit, but it can reduce the stack usage.
After this, stack usage is 672bytes
objdump -d vmlinux | ./scripts/ ppc64 | grep seq_show
0xc0000000007f7cc0 netstat_seq_show_tcpext.isra.3 [vmlinux]:672

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index c6ee8a2..cc41590 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -486,22 +486,37 @@ static const struct file_operations snmp_seq_fops = {
static int netstat_seq_show_tcpext(struct seq_file *seq, void *v)
- int i;
- unsigned long buff[LINUX_MIB_MAX];
+ int i, c;
+ unsigned long buff[LINUX_MIB_MAX/2 + 1];
struct net *net = seq->private;

- memset(buff, 0, sizeof(unsigned long) * LINUX_MIB_MAX);
+ memset(buff, 0, sizeof(unsigned long) * (LINUX_MIB_MAX/2 + 1));

seq_puts(seq, "TcpExt:");
for (i = 0; snmp4_net_list[i].name; i++)
seq_printf(seq, " %s", snmp4_net_list[i].name);

seq_puts(seq, "\nTcpExt:");
- snmp_get_cpu_field_batch(buff, snmp4_net_list,
- net->mib.net_statistics);
- for (i = 0; snmp4_net_list[i].name; i++)
+ for_each_possible_cpu(c) {
+ for (i = 0; i < LINUX_MIB_MAX/2; i++)
+ buff[i] += snmp_get_cpu_field(
+ net->mib.net_statistics,
+ c, snmp4_net_list[i].entry);
+ }
+ for (i = 0; i < LINUX_MIB_MAX/2; i++)
seq_printf(seq, " %lu", buff[i]);

+ memset(buff, 0, sizeof(unsigned long) * (LINUX_MIB_MAX/2 + 1));
+ for_each_possible_cpu(c) {
+ for (i = LINUX_MIB_MAX/2; snmp4_net_list[i].name; i++)
+ buff[i - LINUX_MIB_MAX/2] += snmp_get_cpu_field(
+ net->mib.net_statistics,
+ c,
+ snmp4_net_list[i].entry);
+ }
+ for (i = LINUX_MIB_MAX/2; snmp4_net_list[i].name; i++)
+ seq_printf(seq, " %lu", buff[i - LINUX_MIB_MAX/2]);
return 0;
Yep, it halves the stack usage, but it doesn't look good heh

But well, you may try to post the patchset (with or without this last
change, you pick) officially and see how it goes. As you're posting as
RFC, it's not being evaluated as seriously.
Thanks for the suggestion, I will remove it in future patch version

FWIW, I tested your patches, using your test and /proc/net/snmp file on
a x86_64 box, Intel(R) Xeon(R) CPU E5-2643 v3.

Before the patches:

Performance counter stats for './test /proc/net/snmp':

5.225 cache-misses
12.708.673.785 L1-dcache-loads
1.288.450.174 L1-dcache-load-misses # 10,14% of all L1-dcache hits
1.271.857.028 LLC-loads
4.122 LLC-load-misses # 0,00% of all LL-cache hits

9,174936524 seconds time elapsed


Performance counter stats for './test /proc/net/snmp':

2.865 cache-misses
30.203.883.807 L1-dcache-loads
1.215.774.643 L1-dcache-load-misses # 4,03% of all L1-dcache hits
1.181.662.831 LLC-loads
2.685 LLC-load-misses # 0,00% of all LL-cache hits

13,374445056 seconds time elapsed

Numbers were steady across multiple runs.

Yes, I guess your X86 machine doesn't have the large cpu number as mine (cpu#=160).
The cache misses rate difference btw before and after this patch will be more
significant if the cpu number is large.


+static int netstat_seq_show_ipext(struct seq_file *seq, void *v)
+ int i;
+ u64 buff64[IPSTATS_MIB_MAX];
+ struct net *net = seq->private;
seq_puts(seq, "\nIpExt:");
for (i = 0; snmp4_ipextstats_list[i].name != NULL; i++)
seq_printf(seq, " %s", snmp4_ipextstats_list[i].name);
seq_puts(seq, "\nIpExt:");
You're missing a memset() call here.
Not sure if you missed this one or not..
indeed, thanks

To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at