Full quote for Debian Bug Tracking System bugnumber 504495
( http://bugs.debian.org/504495 )
Op 20090503 om 00:58 schreef Peter Iannucci:
> H. Peter Anvin wrote:
>> Peter Iannucci wrote:
>>
>>> Hello hpa,
>>>
>>> We are developing a Xen-based virtualization service, and we are trying
>>> to reduce excessive CPU utilization at isolinux boot prompts. When
>>> users boot up a virtual machine on an isolinux boot CD, the CPU starts
>>> spinning at 100%. These machines frequently get left in this state for
>>> days or weeks, degrading the service for everyone else.
>>>
>>> You replied to the work-around discussed in the earlier thread at
>>>
>>> http://syslinux.zytor.com/archives/2005-June/005291.html
>>>
>>>
Breaks serial console.
-hpa
>>> In our case, the serial console is not an issue. We would like to
>>> submit a patch to isolinux which HLTs on getchar if isolinux is not
>>> running with the serial console option. Do you think this is the right
>>> solution to this problem? Can we work with you to resolve this?
>>>
>>>
>>
>> It would *probably* be okay to do when serial console is not enabled,
>> but it seems a bit crude... and also it's somewhat confusing that one
>> should enable serial console and get drastically different behavior.
>>
>> What is much more confusing to me is why do you have VMs left like this
>> for "days or weeks"?
It is about wasted resources.
That used to be a non-issue, but now on virtualisation we have to pay ...
>> -hpa
>>
> One improvement might be enabling interrupts for the serial port. On
> line 1277 of 2.6.29/drivers/serial/8250.c, this is accomplished with
>
> serial_outp(up, UART_IER, 0x0f);
>
> where the UART Interrupt Enable Register is defined on line 23 of
> include/linux/serial_reg.h, and serial_outp is responsible for
> special-casing unusual devices. The protocol for enabling interrupts
> via this register is common to all standard PC serial ports (and can
> easily be scraped from 8250.c for exceptions as desired).
>
> On scanning through syslinux/core/parseconfig.inc, it looks like the
> infrastructure for enabling serial IRQ's is already present, but is not
> being used. Once these interrupts are enabled, the HLT should take care
> of both cases fine.
Proposal:
Introduce a configuration keyword named like 'halt_on_idle'.
'halt_on_idle' is optional
and when set is HLT used in the "idle loop" that waits for a key stroke.
> By way of explanation, when our users boot a VM with a live CD and
> reboot from within the guest, the domain still has the CD still
> attached. This leaves the machine spinning at the boot prompt. Users
> forget and leave their machines in this state frequently enough that it
> has become a problem. Similarly, users occasionally create a new VM and
> specify a first-time boot CD, but never log in to complete the install.
> It only takes a few VMs using 100% of a CPU to degrade the performance
> of a host.
Probably should 'halt_on_idle' be default
and is a 'no_halt_on_idle' or a 'active_poll' needed.
'active_poll' has to be mentioned in de section 'serial port' of the docs.
> -Peter Iannucci
Cheers
Geert Stappers
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