lkml.org 
[lkml]   [2010]   [Apr]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectA few questions and issues with dynticks, NOHZ and powertop
Hey!

Before I'm off hiding some Easter eggs, here are some questions and
issues related to "dynticks", NOHZ, and powertop:

1) single-CPU systems, SMP-capable kernel and RCU
2) dual-core CPU[*] and select_nohz_load_balancer()
3) USB, autosuspend failure, excessive ticks
4) SynPS/2 touchpad and hundreds of IRQs per second
5) powertop: 1 + 1 = 1


1) single-CPU systems, SMP-capable kernel and RCU

CONFIG_TREE_RCU=y
CONFIG_RCU_FANOUT=64
CONFIG_RCU_FAST_NO_HZ=y

Booting a SMP-capable kernel with "nosmp", or manually offlining one CPU
(or -- though I haven't tested it -- booting a SMP-capable kernel on a
system with merely one CPU) means that in up to about half of the calls to
tick_nohz_stop_sched_tick() are aborted due to rcu_needs_cpu(). This is
quite strange to me: AFAIK, RCU is an excellent tool for SMP, but not really
needed for UP? And all updates seem to be local to the CPU anyway.
Therefore, I'd presume that rcu_needs_cpu() should return 0 on
one-CPU-systems. Or could RCU switch between TINY_RCU on UP and TREE_RCU on
SMP (using alternatives or whatever)?


2) dual-core CPU[*] and select_nohz_load_balancer()
[*] (Intel(R) Core(TM)2 Duo CPU T7250)

# CONFIG_SCHED_SMT is not set
CONFIG_SCHED_MC=y
CONFIG_SCHED_HRTICK=y

CONFIG_SCHED_MC is igored, as mc_capable() returns 0 on a one-socket,
dual-core system. Quite surprisingly, even under moderate load (~98.0% idle)
while writing this bugreport, up to half of the calls to
tick_nohz_stop_sched_tick() are aborted due to select_nohz_load_balancer(1):

if (atomic_read(&nohz.load_balancer) == -1) {
/* make me the ilb owner */
if (atomic_cmpxchg(&nohz.load_balancer, -1, cpu) == -1)
return 1;

I'm not really sure, but I guess this is caused by the following phenomenon
under minor load but still, every once in a while, parallel work for both
CPUs:

CPU #0 CPU #1

<active> <active>
<idle> <active>
tick_nohz_stop_sched_tick(1) <active>
select_nohz_load_balancer(1) <active>
=> becomes ilb owner <idle>
=> tick is not stopped tick_nohz_stop_sched_tick(1)
=> CPU goes to sleep for 1 tick => as it isn't the ILB owner, tick
<sleep for 1 tick> is stopped .
---> scheduler_tick() <sleeeeeeeep>
tick_nohz_stop_sched_tick(0)
<still idle>
tick_nohz_stop_sched_tick(1)
select_nohz_load_balancer(1)
=> is ilb owner, all CPUs idle,
may go to sleep.

If both CPUs have hardly anything to do, letting the _active_ CPU do ilb
allows us to enter deep sleep states earlier, and longer:

current ILB model (* = ILB)

tick ---------- tick -------- tick ----- IRQ
CPU0: active|IDLE(C2)--|*|IDLE (C3) |
CPU1: active....| IDLE (C3) |
core: .......???| C2 | C3 |

ILB-by-active-CPU-on-light-load:

tick ---------- tick -------- tick ----- IRQ
CPU0: active|IDLE(C3) |
CPU1: active....*| IDLE (C3) |
core: .......????| C3 |


3) USB: built-in UHCI and a built-in 0a5c:2101 Broadcom Corp. A-Link
BlueUsbA2 Bluetooth module; built-in EHCI and a built-in 0ac8:c302 Z-Star
Microelectronics Corp. Vega USB 2.0 Camera.

usbcore.autosuspend is enabled (= 2), of course.

Recent USB suspend statistics
Active Device name
100.0% USB device 7-1 : BCM92045NMD (Broadcom Corp)
100.0% USB device 1-2 : Vega USB 2.0 Camera. (Vimicro Corp.)
100.0% USB device usb7 : UHCI Host Controller (Linux 2.6.34-rc3 uhci_hcd)
100.0% USB device usb1 : EHCI Host Controller (Linux 2.6.34-rc3 ehci_hcd)

Booting into /bin/bash on a SMP kernel booted with "nosmp" leads to ~ 10
wakeups per second; disabling the cursor helps halfway (~ 5 wakeups); and
manually unbinding the USB host drivers from the USB host devices finally
lead to ~ 1.1 wakeups per second. What's keeping USB from suspending these
unused devices here?


4) SynPS/2 touchpad:
Why does moving the touchpad lead to sooo many IRQs? I can't look as fast
as the mouse pointer seems to get new data:
62,5% (473,1) <interrupt> : PS/2 keyboard/mouse/touchpad


5) powertop and hrtimer_start_range_ns (tick_sched_timer) on a SMP kernel
booted with "nosmp":

Wakeups-from-idle per second : 9.9 interval: 15.0s
...
48.5% ( 9.4) <kernel core> : hrtimer_start (tick_sched_timer)
26.1% ( 5.1) <kernel core> : cursor_timer_handler (cursor_timer_handler)
20.6% ( 4.0) <kernel core> : usb_hcd_poll_rh_status (rh_timer_func)
1.0% ( 0.2) <kernel core> : arm_supers_timer (sync_supers_timer_fn)
0.7% ( 0.1) <interrupt> : ata_piix
...

Accoding to http://www.linuxpowertop.org , the count in the brackets is how
many wakeups per seconds were caused by one source. Adding all _except_
48.5% ( 9.4) <kernel core> : hrtimer_start (tick_sched_timer)
up leads to the 9.9; adding also the 9.4 leads to 19.3 wakeups-from-idle per
second. However, http://www.linuxpowertop.org says:

> "Should "Wakeups-from-idle per second" equal the sum of the
> wakeups/second/core listed on the "Top causes for wakeups" list?
>
> It should be higher, since there are some causes for wakeups that are nearly
> impossible to detect by software."


Best, and Happy Easter,

Dominik


\
 
 \ /
  Last update: 2010-04-04 00:37    [W:0.063 / U:0.204 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site