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

From: Marcelo <[email-masked]>
Date: Wed, 21 Sep 2016 15:24:38 -0300

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
> > > B.R.
> > 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/checkstack.pl 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.

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

After:

 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.

  Marcelo

> 
> > > > > +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
> B.R.
> Jia
> > Thanks,
> > Marcelo
> > 
> 
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