On Mon, Oct 22, 2012 at 7:30 PM, Robert P. J. Day <rpjday@xxxxxxxxxxxxxx> wrote:
>
> poking around under /proc on my quad-core asus laptop and just
> noticed that while /proc/cpuinfo (properly) lists my 8 processors,
> /proc/softirqs instead lists 16, with the last 8 having values of all
> zeroes (not surprisingly). with the middle columns snipped for
> brevity, my /proc/softirqs shows:
>
> CPU0 CPU1 ... CPU13 CPU14 CPU15
> HI: 0 0 ... 0 0 0
> TIMER: 1018887 45313 ... 0 0 0
> NET_TX: 1182 1813 ... 0 0 0
> NET_RX: 991 719 ... 0 0 0
> BLOCK: 44559 13 ... 0 0 0
> BLOCK_IOPOLL: 0 0 ... 0 0 0
> TASKLET: 41582 419 ... 0 0 0
> SCHED: 76438 28658 ... 0 0 0
> HRTIMER: 738 709 ... 0 0 0
> RCU: 136797 82392 ... 0 0 0
>
> with all of the zero values from CPU8-CPU15. so the obvious question
> is -- why? why the difference in the way those two proc files count
> the "number" of CPUs on my system?
>
> for /proc/cpuinfo, the logic is in arch/x86/kernel/cpu/proc.c, and
> the way the seq_file is implemented:
>
> static void *c_start(struct seq_file *m, loff_t *pos)
> {
> *pos = cpumask_next(*pos - 1, cpu_online_mask);
> if ((*pos) < nr_cpu_ids)
> return &cpu_data(*pos);
> return NULL;
> }
>
> so that loop clearly iterates through the "online" CPUs, which would
> appear to be the correct loop criteria.
>
> the code for softirqs, however, is in fs/proc/softirqs.c, and is
> much simpler:
>
> static int show_softirqs(struct seq_file *p, void *v)
> {
> int i, j;
>
> seq_puts(p, " ");
> for_each_possible_cpu(i)
> seq_printf(p, "CPU%-8d", i);
> seq_putc(p, '\n');
>
> for (i = 0; i < NR_SOFTIRQS; i++) {
> seq_printf(p, "%12s:", softirq_to_name[i]);
> for_each_possible_cpu(j)
> seq_printf(p, " %10u", kstat_softirqs_cpu(i, j));
> seq_putc(p, '\n');
> }
> return 0;
> }
>
> note that that code uses the macro "for_each_possible_cpu()" rather
> than examining only *online* CPUs. in dmesg, i see the lines:
>
> [ 0.000000] smpboot: Allowing 16 CPUs, 8 hotplug CPUs
> [ 0.000000] setup_percpu: NR_CPUS:256 nr_cpumask_bits:256 nr_cpu_ids:16 nr_node_ids:1
>
> and the code for all this counting is in arch/x86/kernel/smpboot.c,
> which you can check out for yourself, but this brings me back to the
> basic question -- why is the code for softirqs iterating through all
> *possible* CPUs (in my case, apparently, 16), when i have only 8
> *online* CPUs?
Sorry for jumping in but i have basic question here.
What is the difference between *online* CPUs and *possible* CPUs?
Thanks,
VIjay
>
> rday
>
> --
>
> ========================================================================
> Robert P. J. Day Ottawa, Ontario, CANADA
> http://crashcourse.ca
>
> Twitter: http://twitter.com/rpjday
> LinkedIn: http://ca.linkedin.com/in/rpjday
> ========================================================================
>
>
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