[libvirt] [PATCH 2/3] qemu_driver: clamp value for setting sched cpu_shares with --config.

[Dongsheng Yang]

On 05/15/2014 09:11 PM, Ján Tomko wrote:
> On 05/15/2014 11:39 AM, Dongsheng Yang wrote:
>> As shown in 'man virsh' about schedinfo:
>>
>>             Note: The cpu_shares parameter has a valid value range of 0-262144;
>> 	   Negative values are wrapped to positive, and larger values are capped at
>>             the maximum.  Therefore, -1 is a useful shorthand for 262144.
>> 	   On the Linux kernel, the values 0 and 1 are automatically converted to
>> 	   a minimal value of 2.
>> it works well with --live, but not with --config.
>>
>> Example:
>> 	# virsh schedinfo rhel7-ful --set cpu_shares=0 --config
>> 		Scheduler      : posix
>> 		cpu_shares     : 0
>> 		vcpu_period    : 0
>> 		vcpu_quota     : 0
>> 		emulator_period: 0
>> 		emulator_quota : 0
>> cpu_shares is 0 rather than expected 2.
>>
>> What's worse, when we start it again, it is the default value of
>> cpu_shares 1024.
>>
>> Because when we set the value of cpu_shares, when flag is --live,
>> write the value into cgroup/cpu.shares. Then it will convert the
>> value into the range of [2, 262144]. When flag is --config, we
>> set the value into vmdef immidiately and 0 means no settting for
>> cpu_shares. When we start vm again, libvirt use default value(1024)
>> for it.
> commit bdffab0d5c52d31bd71422b0b69665efb6650953
> Author:     Ján Tomko <jtomko at redhat.com>
> CommitDate: 2014-03-26 10:10:02 +0100
>
>      Treat zero cpu shares as a valid value
>
> changed this behavior.
>
> After this commit, if you set cpu_shares to 0 via schedinfo --config, the 0
> will be written in the XML and used on domain startup.

Indeed , it works.
>
> To use the default value, the <shares> element needs to be removed from the
> XML before restarting the domain, this cannot be currently done via 'virsh
> schedinfo'.

And yes, my patch here is attempting to do the similar work in 'virsh 
schedinfo'.

Also, it can make the other "invalid" input, such as -1 and 262144+1, 
get the
expected output as manpage described.
>
> Jan
>