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

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Date:	Thu, 11 Apr 2013 07:08:29 -0500
From:	Robin Holt <holt@....com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Robin Holt <holt@....com>, 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.

On Thu, Apr 11, 2013 at 07:03:58AM -0500, Robin Holt wrote:
> On Thu, Apr 11, 2013 at 02:00:27PM +0200, Ingo Molnar wrote:
> > 
> > * 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
> 
> I used perf record -a /sbin/reboot -f -d -n

OK.  Looking at Russ' patch, I understand now why it is looking idle.
We are still serially doing the DOWN_PREPARE, etc.  All those other cpus
are still sitting idle.

Can we call the __cpu_down functions from an smp_call_function()?

Robin
> 
> Robin
> > 
> > ... 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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