> > I'm proposing to make 'reboot' overhead profilable, via a debug hack:
> >
> > echo 1 > /proc/sys/kernel/magic_dont_fully_reboot_flag
> >
> > perf record reboot
> >
> > perf is using NMIs to profile - and since much of cpu_down() is with irqs
> > disabled, NMI profiling would be needed to see inside the overhead.
> >
> > (Assuming the 240 msecs is CPU overhead, not waiting for some IRQ/IPI
> > event.)
I had the machine booted as 512 cpus.
I tweaked the kernel like this:
diff --git a/kernel/sys.c b/kernel/sys.c
index 39c9c4a..b42bd4f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -368,8 +368,10 @@ EXPORT_SYMBOL(unregister_reboot_notifier);
*/
void kernel_restart(char *cmd)
{
- kernel_restart_prepare(cmd);
+ // kernel_restart_prepare(cmd);
disable_nonboot_cpus();
+ enable_nonboot_cpus();
+ return;
if (!cmd)
printk(KERN_EMERG "Restarting system.\n");
else
perf record -a /sbin/reboot -d -f -n
The top of 'perf report' has:
Events: 14M cycles
22.58% swapper [kernel.kallsyms] [k]
update_cfs_rq_blocked_load
10.52% swapper [kernel.kallsyms] [k] load_balance
4.96% swapper [kernel.kallsyms] [k] ktime_get
4.12% swapper [kernel.kallsyms] [k]
update_blocked_averages
3.55% swapper [kernel.kallsyms] [k] idle_cpu
1.97% swapper [kernel.kallsyms] [k] uv_read_rtc
0.98% swapper [kernel.kallsyms] [k] rcu_process_gp_end
0.84% swapper [kernel.kallsyms] [k]
apic_timer_interrupt
0.84% swapper [kernel.kallsyms] [k] __lock_text_start
0.84% swapper [kernel.kallsyms] [k]
_raw_spin_lock_irqsave
0.73% swapper [kernel.kallsyms] [k] native_safe_halt
0.56% swapper [kernel.kallsyms] [k] rcu_check_callbacks
0.56% swapper [kernel.kallsyms] [k]
native_write_msr_safe
0.44% swapper [kernel.kallsyms] [k] cpumask_next_and
0.42% reboot [kernel.kallsyms] [k]
kmem_cache_alloc_node
The perf data is 676 MB. I don't know how well it compresses, but the
lzma task has been running for a while.
Thanks,
Robin
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