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[PATCH v5 0/7] Reduce cache miss for snmp_fold_field

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  [PATCH v5 0/7] Reduce cache miss for snmp_fold_field Jia He <hejianet@gmail.com> - 2016-09-28 08:30 +0200
    [PATCH v5 6/7] ipv6: Remove useless parameter in __snmp6_fill_statsdev Jia He <hejianet@gmail.com> - 2016-09-28 08:30 +0200
    [PATCH v5 5/7] proc: Reduce cache miss in xfrm_statistics_seq_show Jia He <hejianet@gmail.com> - 2016-09-28 08:30 +0200
    [PATCH v5 7/7] net: Suppress the "Comparison to NULL could be written" warnings Jia He <hejianet@gmail.com> - 2016-09-28 08:30 +0200
    [PATCH v5 3/7] proc: Reduce cache miss in snmp6_seq_show Jia He <hejianet@gmail.com> - 2016-09-28 08:30 +0200
    Re: [PATCH v5 0/7] Reduce cache miss for snmp_fold_field David Miller <davem@davemloft.net> - 2016-09-28 11:40 +0200
      Re: [PATCH v5 0/7] Reduce cache miss for snmp_fold_field hejianet <hejianet@gmail.com> - 2016-09-28 15:50 +0200
        Local variable ordering (was Re: [PATCH v5 0/7] Reduce cache miss for  snmp_fold_field) Edward Cree <ecree@solarflare.com> - 2016-09-28 18:00 +0200

#1492280 — [PATCH v5 0/7] Reduce cache miss for snmp_fold_field

In a PowerPc server with large cpu number(160), besides commit
a3a773726c9f ("net: Optimize snmp stat aggregation by walking all
the percpu data at once"), I watched several other snmp_fold_field
callsites which would cause high cache miss rate.

test source code:
================
My simple test case, which read from the procfs items endlessly:
/***********************************************************/
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define LINELEN  2560
int main(int argc, char **argv)
{
        int i;
        int fd = -1 ;
        int rdsize = 0;
        char buf[LINELEN+1];

        buf[LINELEN] = 0;
        memset(buf,0,LINELEN);

        if(1 >= argc) {
                printf("file name empty\n");
                return -1;
        }

        fd = open(argv[1], O_RDWR, 0644);
        if(0 > fd){
                printf("open error\n");
                return -2;
        }

        for(i=0;i<0xffffffff;i++) {
                while(0 < (rdsize = read(fd,buf,LINELEN))){
                        //nothing here
                }

                lseek(fd, 0, SEEK_SET);
        }

        close(fd);
        return 0;
}
/**********************************************************/

compile and run:
================
gcc test.c -o test

perf stat -d -e cache-misses ./test /proc/net/snmp
perf stat -d -e cache-misses ./test /proc/net/snmp6
perf stat -d -e cache-misses ./test /proc/net/sctp/snmp
perf stat -d -e cache-misses ./test /proc/net/xfrm_stat

before the patch set:
====================
 Performance counter stats for 'system wide':

         355911097      cache-misses                                                 [40.08%]
        2356829300      L1-dcache-loads                                              [60.04%]
         355642645      L1-dcache-load-misses     #   15.09% of all L1-dcache hits   [60.02%]
         346544541      LLC-loads                                                    [59.97%]
            389763      LLC-load-misses           #    0.11% of all LL-cache hits    [40.02%]

       6.245162638 seconds time elapsed

After the patch set:
===================
 Performance counter stats for 'system wide':

         194992476      cache-misses                                                 [40.03%]
        6718051877      L1-dcache-loads                                              [60.07%]
         194871921      L1-dcache-load-misses     #    2.90% of all L1-dcache hits   [60.11%]
         187632232      LLC-loads                                                    [60.04%]
            464466      LLC-load-misses           #    0.25% of all LL-cache hits    [39.89%]

       6.868422769 seconds time elapsed
The cache-miss rate can be reduced from 15% to 2.9%

changelog
=========
v5:
- order local variables from longest to shortest line
v4:
- move memset into one block of if statement in snmp6_seq_show_item
- remove the changes in netstat_seq_show considerred the stack usage is too large
v3:
- introduce generic interface (suggested by Marcelo Ricardo Leitner)
- use max_t instead of self defined macro (suggested by David Miller)
v2:
- fix bug in udplite statistics.
- snmp_seq_show is split into 2 parts

Jia He (7):
  net:snmp: Introduce generic interfaces for snmp_get_cpu_field{,64}
  proc: Reduce cache miss in snmp_seq_show
  proc: Reduce cache miss in snmp6_seq_show
  proc: Reduce cache miss in sctp_snmp_seq_show
  proc: Reduce cache miss in xfrm_statistics_seq_show
  ipv6: Remove useless parameter in __snmp6_fill_statsdev
  net: Suppress the "Comparison to NULL could be written" warnings

 include/net/ip.h     |  23 ++++++++++++
 net/ipv4/proc.c      | 100 +++++++++++++++++++++++++++++++--------------------
 net/ipv6/addrconf.c  |  12 +++----
 net/ipv6/proc.c      |  32 ++++++++++++-----
 net/sctp/proc.c      |  12 ++++---
 net/xfrm/xfrm_proc.c |  12 +++++--
 6 files changed, 131 insertions(+), 60 deletions(-)

-- 
2.5.5

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#1492281 — [PATCH v5 6/7] ipv6: Remove useless parameter in __snmp6_fill_statsdev

The parameter items(always ICMP6_MIB_MAX) is useless for __snmp6_fill_statsdev.

Signed-off-by: Jia He <hejianet@gmail.com>
---
 net/ipv6/addrconf.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 2f1f5d4..35d4baa 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -4961,18 +4961,18 @@ static inline size_t inet6_if_nlmsg_size(void)
 }
 
 static inline void __snmp6_fill_statsdev(u64 *stats, atomic_long_t *mib,
-				      int items, int bytes)
+					int bytes)
 {
 	int i;
-	int pad = bytes - sizeof(u64) * items;
+	int pad = bytes - sizeof(u64) * ICMP6_MIB_MAX;
 	BUG_ON(pad < 0);
 
 	/* Use put_unaligned() because stats may not be aligned for u64. */
-	put_unaligned(items, &stats[0]);
-	for (i = 1; i < items; i++)
+	put_unaligned(ICMP6_MIB_MAX, &stats[0]);
+	for (i = 1; i < ICMP6_MIB_MAX; i++)
 		put_unaligned(atomic_long_read(&mib[i]), &stats[i]);
 
-	memset(&stats[items], 0, pad);
+	memset(&stats[ICMP6_MIB_MAX], 0, pad);
 }
 
 static inline void __snmp6_fill_stats64(u64 *stats, void __percpu *mib,
@@ -5005,7 +5005,7 @@ static void snmp6_fill_stats(u64 *stats, struct inet6_dev *idev, int attrtype,
 				     offsetof(struct ipstats_mib, syncp));
 		break;
 	case IFLA_INET6_ICMP6STATS:
-		__snmp6_fill_statsdev(stats, idev->stats.icmpv6dev->mibs, ICMP6_MIB_MAX, bytes);
+		__snmp6_fill_statsdev(stats, idev->stats.icmpv6dev->mibs, bytes);
 		break;
 	}
 }
-- 
2.5.5

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#1492282 — [PATCH v5 5/7] proc: Reduce cache miss in xfrm_statistics_seq_show

This is to use the generic interfaces snmp_get_cpu_field{,64}_batch to
aggregate the data by going through all the items of each cpu sequentially.

Signed-off-by: Jia He <hejianet@gmail.com>
---
 net/xfrm/xfrm_proc.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/xfrm/xfrm_proc.c b/net/xfrm/xfrm_proc.c
index 9c4fbd8..dc0cea6 100644
--- a/net/xfrm/xfrm_proc.c
+++ b/net/xfrm/xfrm_proc.c
@@ -50,12 +50,18 @@ static const struct snmp_mib xfrm_mib_list[] = {
 
 static int xfrm_statistics_seq_show(struct seq_file *seq, void *v)
 {
-	struct net *net = seq->private;
 	int i;
+	struct net *net = seq->private;
+	unsigned long buff[LINUX_MIB_XFRMMAX];
+
+	memset(buff, 0, sizeof(unsigned long) * LINUX_MIB_XFRMMAX);
+
+	snmp_get_cpu_field_batch(buff, xfrm_mib_list,
+				 net->mib.xfrm_statistics);
 	for (i = 0; xfrm_mib_list[i].name; i++)
 		seq_printf(seq, "%-24s\t%lu\n", xfrm_mib_list[i].name,
-			   snmp_fold_field(net->mib.xfrm_statistics,
-					   xfrm_mib_list[i].entry));
+						buff[i]);
+
 	return 0;
 }
 
-- 
2.5.5

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#1492283 — [PATCH v5 7/7] net: Suppress the "Comparison to NULL could be written" warnings

This is to suppress the checkpatch.pl warning "Comparison to NULL
could be written". No functional changes here.

Signed-off-by: Jia He <hejianet@gmail.com>
---
 net/ipv4/proc.c | 32 ++++++++++++++++----------------
 net/sctp/proc.c |  2 +-
 2 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index e01f4df..1a7a773 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -357,22 +357,22 @@ static void icmp_put(struct seq_file *seq)
 	atomic_long_t *ptr = net->mib.icmpmsg_statistics->mibs;
 
 	seq_puts(seq, "\nIcmp: InMsgs InErrors InCsumErrors");
-	for (i = 0; icmpmibmap[i].name != NULL; i++)
+	for (i = 0; icmpmibmap[i].name; i++)
 		seq_printf(seq, " In%s", icmpmibmap[i].name);
 	seq_puts(seq, " OutMsgs OutErrors");
-	for (i = 0; icmpmibmap[i].name != NULL; i++)
+	for (i = 0; icmpmibmap[i].name; i++)
 		seq_printf(seq, " Out%s", icmpmibmap[i].name);
 	seq_printf(seq, "\nIcmp: %lu %lu %lu",
 		snmp_fold_field(net->mib.icmp_statistics, ICMP_MIB_INMSGS),
 		snmp_fold_field(net->mib.icmp_statistics, ICMP_MIB_INERRORS),
 		snmp_fold_field(net->mib.icmp_statistics, ICMP_MIB_CSUMERRORS));
-	for (i = 0; icmpmibmap[i].name != NULL; i++)
+	for (i = 0; icmpmibmap[i].name; i++)
 		seq_printf(seq, " %lu",
 			   atomic_long_read(ptr + icmpmibmap[i].index));
 	seq_printf(seq, " %lu %lu",
 		snmp_fold_field(net->mib.icmp_statistics, ICMP_MIB_OUTMSGS),
 		snmp_fold_field(net->mib.icmp_statistics, ICMP_MIB_OUTERRORS));
-	for (i = 0; icmpmibmap[i].name != NULL; i++)
+	for (i = 0; icmpmibmap[i].name; i++)
 		seq_printf(seq, " %lu",
 			   atomic_long_read(ptr + (icmpmibmap[i].index | 0x100)));
 }
@@ -389,7 +389,7 @@ static int snmp_seq_show_ipstats(struct seq_file *seq, void *v)
 	memset(buff64, 0, IPSTATS_MIB_MAX * sizeof(u64));
 
 	seq_puts(seq, "Ip: Forwarding DefaultTTL");
-	for (i = 0; snmp4_ipstats_list[i].name != NULL; i++)
+	for (i = 0; snmp4_ipstats_list[i].name; i++)
 		seq_printf(seq, " %s", snmp4_ipstats_list[i].name);
 
 	seq_printf(seq, "\nIp: %d %d",
@@ -400,7 +400,7 @@ static int snmp_seq_show_ipstats(struct seq_file *seq, void *v)
 	snmp_get_cpu_field64_batch(buff64, snmp4_ipstats_list,
 				   net->mib.ip_statistics,
 				   offsetof(struct ipstats_mib, syncp));
-	for (i = 0; snmp4_ipstats_list[i].name != NULL; i++)
+	for (i = 0; snmp4_ipstats_list[i].name; i++)
 		seq_printf(seq, " %llu", buff64[i]);
 
 	return 0;
@@ -415,13 +415,13 @@ static int snmp_seq_show_tcp_udp(struct seq_file *seq, void *v)
 	memset(buff, 0, TCPUDP_MIB_MAX * sizeof(unsigned long));
 
 	seq_puts(seq, "\nTcp:");
-	for (i = 0; snmp4_tcp_list[i].name != NULL; i++)
+	for (i = 0; snmp4_tcp_list[i].name; i++)
 		seq_printf(seq, " %s", snmp4_tcp_list[i].name);
 
 	seq_puts(seq, "\nTcp:");
 	snmp_get_cpu_field_batch(buff, snmp4_tcp_list,
 				 net->mib.tcp_statistics);
-	for (i = 0; snmp4_tcp_list[i].name != NULL; i++) {
+	for (i = 0; snmp4_tcp_list[i].name; i++) {
 		/* MaxConn field is signed, RFC 2012 */
 		if (snmp4_tcp_list[i].entry == TCP_MIB_MAXCONN)
 			seq_printf(seq, " %ld", buff[i]);
@@ -434,10 +434,10 @@ static int snmp_seq_show_tcp_udp(struct seq_file *seq, void *v)
 	snmp_get_cpu_field_batch(buff, snmp4_udp_list,
 				 net->mib.udp_statistics);
 	seq_puts(seq, "\nUdp:");
-	for (i = 0; snmp4_udp_list[i].name != NULL; i++)
+	for (i = 0; snmp4_udp_list[i].name; i++)
 		seq_printf(seq, " %s", snmp4_udp_list[i].name);
 	seq_puts(seq, "\nUdp:");
-	for (i = 0; snmp4_udp_list[i].name != NULL; i++)
+	for (i = 0; snmp4_udp_list[i].name; i++)
 		seq_printf(seq, " %lu", buff[i]);
 
 	memset(buff, 0, TCPUDP_MIB_MAX * sizeof(unsigned long));
@@ -446,10 +446,10 @@ static int snmp_seq_show_tcp_udp(struct seq_file *seq, void *v)
 	seq_puts(seq, "\nUdpLite:");
 	snmp_get_cpu_field_batch(buff, snmp4_udp_list,
 				 net->mib.udplite_statistics);
-	for (i = 0; snmp4_udp_list[i].name != NULL; i++)
+	for (i = 0; snmp4_udp_list[i].name; i++)
 		seq_printf(seq, " %s", snmp4_udp_list[i].name);
 	seq_puts(seq, "\nUdpLite:");
-	for (i = 0; snmp4_udp_list[i].name != NULL; i++)
+	for (i = 0; snmp4_udp_list[i].name; i++)
 		seq_printf(seq, " %lu", buff[i]);
 
 	seq_putc(seq, '\n');
@@ -492,21 +492,21 @@ static int netstat_seq_show(struct seq_file *seq, void *v)
 	struct net *net = seq->private;
 
 	seq_puts(seq, "TcpExt:");
-	for (i = 0; snmp4_net_list[i].name != NULL; i++)
+	for (i = 0; snmp4_net_list[i].name; i++)
 		seq_printf(seq, " %s", snmp4_net_list[i].name);
 
 	seq_puts(seq, "\nTcpExt:");
-	for (i = 0; snmp4_net_list[i].name != NULL; i++)
+	for (i = 0; snmp4_net_list[i].name; i++)
 		seq_printf(seq, " %lu",
 			   snmp_fold_field(net->mib.net_statistics,
 					   snmp4_net_list[i].entry));
 
 	seq_puts(seq, "\nIpExt:");
-	for (i = 0; snmp4_ipextstats_list[i].name != NULL; i++)
+	for (i = 0; snmp4_ipextstats_list[i].name; i++)
 		seq_printf(seq, " %s", snmp4_ipextstats_list[i].name);
 
 	seq_puts(seq, "\nIpExt:");
-	for (i = 0; snmp4_ipextstats_list[i].name != NULL; i++)
+	for (i = 0; snmp4_ipextstats_list[i].name; i++)
 		seq_printf(seq, " %llu",
 			   snmp_fold_field64(net->mib.ip_statistics,
 					     snmp4_ipextstats_list[i].entry,
diff --git a/net/sctp/proc.c b/net/sctp/proc.c
index 9e7f4a7..16d7f3f 100644
--- a/net/sctp/proc.c
+++ b/net/sctp/proc.c
@@ -81,7 +81,7 @@ static int sctp_snmp_seq_show(struct seq_file *seq, void *v)
 
 	snmp_get_cpu_field_batch(buff, sctp_snmp_list,
 				 net->sctp.sctp_statistics);
-	for (i = 0; sctp_snmp_list[i].name != NULL; i++)
+	for (i = 0; sctp_snmp_list[i].name; i++)
 		seq_printf(seq, "%-32s\t%ld\n", sctp_snmp_list[i].name,
 						buff[i]);
 
-- 
2.5.5

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#1492284 — [PATCH v5 3/7] proc: Reduce cache miss in snmp6_seq_show

This is to use the generic interfaces snmp_get_cpu_field{,64}_batch to
aggregate the data by going through all the items of each cpu sequentially.

Signed-off-by: Jia He <hejianet@gmail.com>
---
 net/ipv6/proc.c | 32 +++++++++++++++++++++++---------
 1 file changed, 23 insertions(+), 9 deletions(-)

diff --git a/net/ipv6/proc.c b/net/ipv6/proc.c
index 679253d0..e047701 100644
--- a/net/ipv6/proc.c
+++ b/net/ipv6/proc.c
@@ -30,6 +30,11 @@
 #include <net/transp_v6.h>
 #include <net/ipv6.h>
 
+#define MAX4(a, b, c, d) \
+	max_t(u32, max_t(u32, a, b), max_t(u32, c, d))
+#define SNMP_MIB_MAX MAX4(UDP_MIB_MAX, TCP_MIB_MAX, \
+			IPSTATS_MIB_MAX, ICMP_MIB_MAX)
+
 static int sockstat6_seq_show(struct seq_file *seq, void *v)
 {
 	struct net *net = seq->private;
@@ -192,13 +197,19 @@ static void snmp6_seq_show_item(struct seq_file *seq, void __percpu *pcpumib,
 				const struct snmp_mib *itemlist)
 {
 	int i;
-	unsigned long val;
-
-	for (i = 0; itemlist[i].name; i++) {
-		val = pcpumib ?
-			snmp_fold_field(pcpumib, itemlist[i].entry) :
-			atomic_long_read(smib + itemlist[i].entry);
-		seq_printf(seq, "%-32s\t%lu\n", itemlist[i].name, val);
+	unsigned long buff[SNMP_MIB_MAX];
+
+	if (pcpumib) {
+		memset(buff, 0, sizeof(unsigned long) * SNMP_MIB_MAX);
+
+		snmp_get_cpu_field_batch(buff, itemlist, pcpumib);
+		for (i = 0; itemlist[i].name; i++)
+			seq_printf(seq, "%-32s\t%lu\n",
+				   itemlist[i].name, buff[i]);
+	} else {
+		for (i = 0; itemlist[i].name; i++)
+			seq_printf(seq, "%-32s\t%lu\n", itemlist[i].name,
+				   atomic_long_read(smib + itemlist[i].entry));
 	}
 }
 
@@ -206,10 +217,13 @@ static void snmp6_seq_show_item64(struct seq_file *seq, void __percpu *mib,
 				  const struct snmp_mib *itemlist, size_t syncpoff)
 {
 	int i;
+	u64 buff64[SNMP_MIB_MAX];
+
+	memset(buff64, 0, sizeof(unsigned long) * SNMP_MIB_MAX);
 
+	snmp_get_cpu_field64_batch(buff64, itemlist, mib, syncpoff);
 	for (i = 0; itemlist[i].name; i++)
-		seq_printf(seq, "%-32s\t%llu\n", itemlist[i].name,
-			   snmp_fold_field64(mib, itemlist[i].entry, syncpoff));
+		seq_printf(seq, "%-32s\t%llu\n", itemlist[i].name, buff64[i]);
 }
 
 static int snmp6_seq_show(struct seq_file *seq, void *v)
-- 
2.5.5

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#1492490

From: Jia He <hejianet@gmail.com>
Date: Wed, 28 Sep 2016 14:22:21 +0800

> v5:
> - order local variables from longest to shortest line

I still see many cases where this problem still exists.  Please
do not resubmit this patch series until you fix all of them.

Patch #2:

-static int snmp_seq_show(struct seq_file *seq, void *v)
+static int snmp_seq_show_ipstats(struct seq_file *seq, void *v)
 {
 	int i;
+	u64 buff64[IPSTATS_MIB_MAX];
 	struct net *net = seq->private;

The order should be "net" then "buff64" then "i".

+static int snmp_seq_show_tcp_udp(struct seq_file *seq, void *v)
+{
+	int i;
+	struct net *net = seq->private;
+	unsigned long buff[TCPUDP_MIB_MAX];

The order should be "buff", "net", then "i".

Patch #3:

@@ -192,13 +197,19 @@ static void snmp6_seq_show_item(struct seq_file *seq, void __percpu *pcpumib,
 				const struct snmp_mib *itemlist)
 {
 	int i;
-	unsigned long val;
-
 ...
+	unsigned long buff[SNMP_MIB_MAX];

The order should be "buff" then "i".

@@ -206,10 +217,13 @@ static void snmp6_seq_show_item64(struct seq_file *seq, void __percpu *mib,
 				  const struct snmp_mib *itemlist, size_t syncpoff)
 {
 	int i;
+	u64 buff64[SNMP_MIB_MAX];

Likewise.

I cannot be any more explicit in my request than this.

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#1492610

On 9/28/16 5:08 PM, David Miller wrote:
> From: Jia He <hejianet@gmail.com>
> Date: Wed, 28 Sep 2016 14:22:21 +0800
>
>> v5:
>> - order local variables from longest to shortest line
> I still see many cases where this problem still exists.  Please
> do not resubmit this patch series until you fix all of them.
>
> Patch #2:
>
> -static int snmp_seq_show(struct seq_file *seq, void *v)
> +static int snmp_seq_show_ipstats(struct seq_file *seq, void *v)
>   {
>   	int i;
> +	u64 buff64[IPSTATS_MIB_MAX];
>   	struct net *net = seq->private;
>
> The order should be "net" then "buff64" then "i".
Sorry for my bad eyesight and quick hand :(
B.R.
Jia
>
> +static int snmp_seq_show_tcp_udp(struct seq_file *seq, void *v)
> +{
> +	int i;
> +	struct net *net = seq->private;
> +	unsigned long buff[TCPUDP_MIB_MAX];
>
> The order should be "buff", "net", then "i".
>
> Patch #3:
>
> @@ -192,13 +197,19 @@ static void snmp6_seq_show_item(struct seq_file *seq, void __percpu *pcpumib,
>   				const struct snmp_mib *itemlist)
>   {
>   	int i;
> -	unsigned long val;
> -
>   ...
> +	unsigned long buff[SNMP_MIB_MAX];
>
> The order should be "buff" then "i".
>
> @@ -206,10 +217,13 @@ static void snmp6_seq_show_item64(struct seq_file *seq, void __percpu *mib,
>   				  const struct snmp_mib *itemlist, size_t syncpoff)
>   {
>   	int i;
> +	u64 buff64[SNMP_MIB_MAX];
>
> Likewise.
>
> I cannot be any more explicit in my request than this.
>

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#1492672 — Local variable ordering (was Re: [PATCH v5 0/7] Reduce cache miss for snmp_fold_field)

On 28/09/16 14:45, hejianet wrote:
>
>
> On 9/28/16 5:08 PM, David Miller wrote:
>> From: Jia He <hejianet@gmail.com>
>> Date: Wed, 28 Sep 2016 14:22:21 +0800
>>
>>> v5:
>>> - order local variables from longest to shortest line
>> I still see many cases where this problem still exists.  Please
>> do not resubmit this patch series until you fix all of them.
>>
>> Patch #2:
>>
>> -static int snmp_seq_show(struct seq_file *seq, void *v)
>> +static int snmp_seq_show_ipstats(struct seq_file *seq, void *v)
>>   {
>>       int i;
>> +    u64 buff64[IPSTATS_MIB_MAX];
>>       struct net *net = seq->private;
>>
>> The order should be "net" then "buff64" then "i".
> Sorry for my bad eyesight and quick hand :(
> B.R.
> Jia
A few months back I wrote a script to help find these:
https://github.com/ecree-solarflare/xmastree
It can check a source file or a patch.
Posting in the hope that people will find it useful.

-Ed

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