linux-kernel - Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer

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Date:	Wed, 06 May 2015 09:33:36 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	viresh kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Miao <realmz6@...il.com>, shashim@...eaurora.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, cl@...nel.org
Subject: Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate
 running timer

On Tue, 2015-05-05 at 15:00 +0200, Thomas Gleixner wrote:
> On Sat, 25 Apr 2015, Eric Dumazet wrote:
> > On Thu, 2015-04-23 at 14:45 +0200, Thomas Gleixner wrote:
> > 
> > > You definitely have a point from the high throughput networking
> > > perspective.
> > > 
> > > Though in a power optimizing scenario with minimal network traffic
> > > this might be the wrong decision. We have to gather data from the
> > > power maniacs whether this matters or not. The FULL_NO_HZ camp might
> > > be pretty unhappy about the above.
> > 
> > Sure, I understand.
> > 
> > 
> > To make this clear, here the profile on a moderately loaded TCP server,
> > pushing ~20Gbits of data. Most of TCP output is ACK clock driven (thus
> > from softirq context).
> > 
> > (using regular sendmsg() system calls, that why the
> > get_nohz_timer_target() is 'only' second in the profile, but add the
> > find_next_bit() to it and this is very close being at first position)
> > 
> > 
> > 
> >    PerfTop:    4712 irqs/sec  kernel:96.7%  exact:  0.0% [4000Hz cycles],  (all, 72 CPUs)
> > -----------------------------------------------------------------------------------------
> >     10.16%  [kernel]          [k] copy_user_enhanced_fast_string            
> >      5.66%  [kernel]          [k] get_nohz_timer_target                     
> >      5.59%  [kernel]          [k] _raw_spin_lock                            
> >      2.53%  [kernel]          [k] __netif_receive_skb_core                  
> >      2.27%  [kernel]          [k] find_next_bit                             
> >      1.90%  [kernel]          [k] tcp_ack                                   
> > 
> > Maybe a reasonable heuristic would be to
> > change /proc/sys/kernel/timer_migration default to 0 on hosts with more
> > than 32 cpus.
> > 
> > profile with timer_migration = 0
> > 
> >    PerfTop:    3656 irqs/sec  kernel:94.3%  exact:  0.0% [4000Hz cycles],  (all, 72 CPUs)
> > -----------------------------------------------------------------------------------------
> >     13.95%  [kernel]          [k] copy_user_enhanced_fast_string            
> >      4.65%  [kernel]          [k] _raw_spin_lock                            
> >      2.57%  [kernel]          [k] __netif_receive_skb_core                  
> >      2.33%  [kernel]          [k] tcp_ack               
> 
> Is that with the static key patch applied?

This was without.

I applied your patch on current linus tree, but for some reason my 72
cpu host is not liking the resulting kernel. I had to ask for a repair,
and this might take a while.

Note your kernel works correctly on other hosts, but with 48 or 32 cpus,
so this must be something unrelated.

I'll let you know when I get more interesting data.

Thanks



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