On Mon, Oct 22, 2012 at 7:30 PM, Robert P. J. Day <rpjday@crashcourse.ca> 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 ========================================================================
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies