[libvirt] [PATCH v2 2/2] qemu: Refresh capabilities when creating resctrl allocation

[Martin Kletzander]

On Fri, Feb 02, 2018 at 06:05:56PM +0100, Peter Krempa wrote:
>On Fri, Feb 02, 2018 at 15:27:22 +0100, Martin Kletzander wrote:
>> Since one of the things in capabilities (info from resctrl updated with data
>> about caches) can be change on the system by remounting the /sys/fs/resctrl with
>> different options, the capabilities need to be refreshed.  There is a better fix
>> in the works, but it's going to be way bigger than this (hence the XXX note
>> there), so for the time being let's workaround this.  And in order not to slow
>> down the domain starting, only get the capabilities if there are any cachetunes.
>>
>> Relates-to: https://bugzilla.redhat.com/show_bug.cgi?id=1540780
>>
>> Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
>> ---
>>  src/qemu/qemu_process.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>> index 5a364730c8c1..2ab242b7634d 100644
>> --- a/src/qemu/qemu_process.c
>> +++ b/src/qemu/qemu_process.c
>> @@ -2514,9 +2514,15 @@ qemuProcessResctrlCreate(virQEMUDriverPtr driver,
>>  {
>>      int ret = -1;
>>      size_t i = 0;
>> -    virCapsPtr caps = virQEMUDriverGetCapabilities(driver, false);
>> +    virCapsPtr caps = NULL;
>>      qemuDomainObjPrivatePtr priv = vm->privateData;
>>
>> +    if (!vm->def->ncachetunes)
>> +        return 0;
>> +
>> +    /* Force capability refresh since resctrl info can change
>> +     * XXX: move cache info into virresctrl so caps are not needed */
>> +    caps = virQEMUDriverGetCapabilities(driver, true);
>
>I hope that your upcoming fix will also deal with the hugepage and NUMa
>data which is more often used than this.
>

Actually, it will just deal with resctrl info being as consistent as
possible.  I mean you can always remount the directory between libvirtd
reading the two files, there will always be a window of opportunity.
The fix I have in mind (and code) is just going to make rid of the
capability usage from the driver in resctrl-related code.  Do you have
any pointers to the numa and hugepage faults that we ought to fix?
Maybe it can be dealt with the same way.

>My feelings towards doing this are slightly mixed, but ACK

Thanks, pinky swear that I'll remove it in near future ;)
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