Re: [pve-devel] [PATCH qemu-server] fix #2570: add 'keephugepages' config

[Stefan Reiter <s.reiter@proxmox.com>]

On 8/20/20 10:17 AM, Thomas Lamprecht wrote:
> On 12.02.20 14:32, Stefan Reiter wrote:
>> We already keep hugepages if they are created with the kernel
>> commandline (hugepagesz=x hugepages=y), but some setups (specifically
>> hugepages across multiple NUMA nodes) cannot be configured that way.
>> Since we always clear these hugepages at VM shutdown, rebooting a VM
>> that uses them might not work, since the requested count might not be
>> available anymore by the time we want to use them (also, we would then
>> no longer allocate them correctly on the NUMA nodes).
>>
> 
> but they also will be kept on normal shutdown now.
> Why not do this transparent, without extra config, solely on API reboot call?
> 

That's the intent, though the reboot API call not releasing them seems 
like an additional fix.

 From the bug report:

"""
Sometimes it's useful to allocate hugepages at boot time to prevent too 
much continuous memory pages being used (eg. cached), so when we start 
the VM we can sure it has enough hugepages.

Currently hugepages are freed when VM stops, it's possible that when we 
need start the VM again, not enough hugepages can be allocated (which is 
my case).

It would be great to have an option to disable hugepage free when VM stop.
"""

It really is an edge case though...

>> Add a 'keephugepages' parameter to skip cleanup and simply leave them
>> untouched.
>>
>> Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
>> ---
>>
>> I tried adding it as a 'keep' sub-parameter first (i.e.
>> 'hugepages: 1024,keep=1' etc.), but it turns out that we hardcode
>> $config->{hugepages} to be a numeric string in a *lot* of different places, so I
>> opted for this variant instead. Open for suggestions ofc.
>>
>>   PVE/QemuServer.pm | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
>> index 23176dd..4741707 100644
>> --- a/PVE/QemuServer.pm
>> +++ b/PVE/QemuServer.pm
>> @@ -384,6 +384,14 @@ EODESC
>>   	description => "Enable/disable hugepages memory.",
>>   	enum => [qw(any 2 1024)],
>>       },
>> +    keephugepages => {
>> +	optional => 1,
>> +	type => 'boolean',
>> +	default => 0,
>> +	description => "Use together with hugepages. If enabled, hugepages will"
>> +		     . " not be deleted after VM shutdown and can be used for"
>> +		     . " subsequent starts.",
>> +    },
>>       vcpus => {
>>   	optional => 1,
>>   	type => 'integer',
>> @@ -5441,11 +5449,13 @@ sub vm_start {
>>   
>>   		eval { $run_qemu->() };
>>   		if (my $err = $@) {
>> -		    PVE::QemuServer::Memory::hugepages_reset($hugepages_host_topology);
>> +		    PVE::QemuServer::Memory::hugepages_reset($hugepages_host_topology)
>> +			if !$conf->{keephugepages};
>>   		    die $err;
>>   		}
>>   
>> -		PVE::QemuServer::Memory::hugepages_pre_deallocate($hugepages_topology);
>> +		PVE::QemuServer::Memory::hugepages_pre_deallocate($hugepages_topology)
>> +		    if !$conf->{keephugepages};
>>   	    };
>>   	    eval { PVE::QemuServer::Memory::hugepages_update_locked($code); };
>>   
>>
>