On Mon, 22 Oct 2012, Vijay Chauhan wrote:
> 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?
that is precisely the confusion i'm having here, so i'm pawing thru
the code as we speak to figure that out.
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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