linux-kernel - Re: [PATCH] Do not force shutdown/reboot to boot cpu.

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Date:	Thu, 11 Apr 2013 14:00:27 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Robin Holt <holt@....com>
Cc:	Russ Anderson <rja@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH] Do not force shutdown/reboot to boot cpu.


* Robin Holt <holt@....com> wrote:

> > Ok, so it looks profilable.
> > 
> > The result above is not surprising: most CPUs sit in idle and don't do anything, 
> > while the loop goes on, right?
> > 
> > The interesting thing to profile would be the parallel bring-down, with the 
> > simplest global lock solution you mentioned. In that case most CPUs should be 
> > doing 'something' all the time - maybe spinning on the lock, maybe something else, 
> > right?
> 
> Again, mostly looks idle.

Forgot to suggest:

  perf record -a /sbin/reboot

... to capture remote CPU activity too.

> Events: 5M cycles
>     31.69%          swapper  [kernel.kallsyms]        [k] update_cfs_rq_blocked_load
>     14.22%          swapper  [kernel.kallsyms]        [k] load_balance
>     12.95%          swapper  [kernel.kallsyms]        [k] ktime_get
>      4.64%          swapper  [kernel.kallsyms]        [k] idle_cpu
>      3.46%          swapper  [kernel.kallsyms]        [k] uv_read_rtc
>      2.26%          swapper  [kernel.kallsyms]        [k] ktime_get_update_offsets
>      2.25%          swapper  [kernel.kallsyms]        [k] rcu_check_callbacks
>      1.72%          swapper  [kernel.kallsyms]        [k] _raw_spin_lock_irqsave
>      1.57%          swapper  [kernel.kallsyms]        [k] native_write_msr_safe
>      1.53%          swapper  [kernel.kallsyms]        [k] native_safe_halt
>      1.52%          swapper  [kernel.kallsyms]        [k] apic_timer_interrupt
>      1.52%          swapper  [kernel.kallsyms]        [k] update_blocked_averages
>      1.51%          swapper  [kernel.kallsyms]        [k] __lock_text_start
>      1.48%          swapper  [kernel.kallsyms]        [k] rcu_process_gp_end
>      1.40%          swapper  [kernel.kallsyms]        [k] rcu_process_callbacks
>      1.19%           reboot  [kernel.kallsyms]        [k] kmem_cache_alloc_node
>      0.63%          swapper  [kernel.kallsyms]        [k] check_for_new_grace_period
>      0.58%          swapper  [kernel.kallsyms]        [k] rebalance_domains
>      0.55%          swapper  [kernel.kallsyms]        [k] cpumask_next_and
>      0.54%          swapper  [kernel.kallsyms]        [k] __tick_nohz_idle_enter
>      0.53%          swapper  [kernel.kallsyms]        [k] perf_adjust_freq_unthr_context
>      0.49%          swapper  [kernel.kallsyms]        [k] _raw_spin_lock

If even perf record -a shows a mostly idle system, then the overhead must be in 
sleep/wakeup latencies - for that the next step would be to figure out where all 
the waiting happens, for example via call-graph context-switch profiling:

   perf stat --null perf record -a -g -e sched:sched_switch /sbin/reboot

(the perf stat --null will tell us the runtime of the whole operation.)

Thanks,

	Ingo
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