linux-kernel - Re: [patch 0/3] kvm tool: Serial emulation overhaul

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Date:	Sun, 11 Dec 2011 11:46:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Sasha Levin <levinsasha928@...il.com>
Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul


btw., i just noticed another performance weirdness, running the 
latest version on -tip with such a 64-bit x86 kernel:

  make defconfig
  make kvmconfig   # sidenote: sigh - this still needs a 'make oldconfig'


and do a 'kvm run' into a shell, then I get a permanently busy 
kvm executable sucking up CPU time:

   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 28206 mingo     20   0 3586m  80m 1080 S 56.1  0.7   0:13.83 kvm
 29462 mingo     20   0 15380 1392  900 R  0.3  0.0   0:00.02 top

perf kvm top suggests simple idling around of 16 CPUs:

--------------------------------------------------------------------------------------------------
   PerfTop:    1635 irqs/sec  kernel:91.1% us: 0.3% guest kernel: 8.5% guest us: 0.0% exact:  0.0% [1000Hz cycles],  (all, 16 CPUs)
--------------------------------------------------------------------------------------------------

             samples  pcnt function                  DSO
             _______ _____ _________________________ _______________________________

              155.00 17.9% apic_timer_interrupt      /home/mingo/linux/linux/vmlinux
               89.00 10.3% find_busiest_group        /home/mingo/linux/linux/vmlinux
               66.00  7.6% native_apic_mem_write     /home/mingo/linux/linux/vmlinux
               51.00  5.9% idle_cpu                  /home/mingo/linux/linux/vmlinux
               29.00  3.3% __rcu_pending             /home/mingo/linux/linux/vmlinux
               25.00  2.9% run_posix_cpu_timers      /home/mingo/linux/linux/vmlinux
               24.00  2.8% _raw_spin_lock            /home/mingo/linux/linux/vmlinux
               19.00  2.2% cpumask_next_and          /home/mingo/linux/linux/vmlinux
               19.00  2.2% ktime_get                 /home/mingo/linux/linux/vmlinux
               18.00  2.1% run_timer_softirq         /home/mingo/linux/linux/vmlinux
               16.00  1.8% run_rebalance_domains     /home/mingo/linux/linux/vmlinux
               16.00  1.8% native_sched_clock        /home/mingo/linux/linux/vmlinux
               15.00  1.7% rcu_exit_nohz             /home/mingo/linux/linux/vmlinux
               14.00  1.6% tick_nohz_stop_sched_tick /home/mingo/linux/linux/vmlinux
               14.00  1.6% hrtimer_run_queues        /home/mingo/linux/linux/vmlinux

NO_HZ is set in the .config.

Btw., 'perf kvm' is in need of some love: for example it is 
unable to pick up a vmlinux from the current directory and 
./vmlinux does not work either, it needs 
/home/mingo/linux/linux/vmlinux specified explicitly - that 
sucks.

Ideally perf could be taught to talk to the guest kvm process 
and get its vmlinux home position (even better: its guest 
kallsyms) from there - so if i typed 'perf kvm top' it would do 
the right thing all automagically. [ What a fantastc level of 
automation! ;-) ]

Thanks,

	Ingo
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