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Re: [libvirt] [PATCH v2] qemu: Introduce caching whether /dev/kvm is accessible



On Thu, Nov 01, 2018 at 04:37 PM +0100, Michal Privoznik <mprivozn redhat com> wrote:
> On 10/30/2018 01:55 PM, Daniel P. Berrangé wrote:
>> On Tue, Oct 30, 2018 at 10:32:08AM +0000, Daniel P. Berrangé wrote:
>>> On Tue, Oct 30, 2018 at 11:08:45AM +0100, Michal Privoznik wrote:
>>>> On 10/30/2018 10:35 AM, Daniel P. Berrangé wrote:
>>>>> On Tue, Oct 30, 2018 at 09:13:50AM +0100, Michal Privoznik wrote:
>>>>>> On 10/29/2018 06:34 PM, Marc Hartmayer wrote:
>>>>>>> Introduce caching whether /dev/kvm is usable as the QEMU user:QEMU
>>>>>>> group. This reduces the overhead of the QEMU capabilities cache
>>>>>>> lookup. Before this patch there were many fork() calls used for
>>>>>>> checking whether /dev/kvm is accessible. Now we store the result
>>>>>>> whether /dev/kvm is accessible or not and we only need to re-run the
>>>>>>> virFileAccessibleAs check if the ctime of /dev/kvm has changed.
>>>>>>>
>>>>>>> Suggested-by: Daniel P. Berrangé <berrange redhat com>
>>>>>>> Signed-off-by: Marc Hartmayer <mhartmay linux ibm com>
>>>>>>> ---
>>>>>>>  src/qemu/qemu_capabilities.c | 54 ++++++++++++++++++++++++++++++++++--
>>>>>>>  1 file changed, 52 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
>>>>>>> index e228f52ec0bb..85516954149b 100644
>>>>>>> --- a/src/qemu/qemu_capabilities.c
>>>>>>> +++ b/src/qemu/qemu_capabilities.c
>>>>>>> @@ -3238,6 +3238,10 @@ struct _virQEMUCapsCachePriv {
>>>>>>>      virArch hostArch;
>>>>>>>      unsigned int microcodeVersion;
>>>>>>>      char *kernelVersion;
>>>>>>> +
>>>>>>> +    /* cache whether /dev/kvm is usable as runUid:runGuid */
>>>>>>> +    virTristateBool kvmUsable;
>>>>>>> +    time_t kvmCtime;
>>>>>>>  };
>>>>>>>  typedef struct _virQEMUCapsCachePriv virQEMUCapsCachePriv;
>>>>>>>  typedef virQEMUCapsCachePriv *virQEMUCapsCachePrivPtr;
>>>>>>> @@ -3824,6 +3828,52 @@ virQEMUCapsSaveFile(void *data,
>>>>>>>  }
>>>>>>>
>>>>>>>
>>>>>>> +/* Determine whether '/dev/kvm' is usable as QEMU user:QEMU group. */
>>>>>>> +static bool
>>>>>>> +virQEMUCapsKVMUsable(virQEMUCapsCachePrivPtr priv)
>>>>>>> +{
>>>>>>> +    struct stat sb;
>>>>>>> +    static const char *kvm_device = "/dev/kvm";
>>>>>>> +    virTristateBool value;
>>>>>>> +    virTristateBool cached_value = priv->kvmUsable;
>>>>>>> +    time_t kvm_ctime;
>>>>>>> +    time_t cached_kvm_ctime = priv->kvmCtime;
>>>>>>> +
>>>>>>> +    if (stat(kvm_device, &sb) < 0) {
>>>>>>> +        virReportSystemError(errno,
>>>>>>> +                             _("Failed to stat %s"), kvm_device);
>>>>>>> +        return false;
>>>>>>> +    }
>>>>>>> +    kvm_ctime = sb.st_ctime;
>>>>>>
>>>>>> This doesn't feel right. /dev/kvm ctime is changed every time qemu is
>>>>>> started or powered off (try running stat over it before and after a
>>>>>> domain is started/shut off). So effectively we will fork more often than
>>>>>> we would think. Should we cache inode number instead? Because for all
>>>>>> that we care is simply if the file is there.
>>>>>
>>>>> Urgh, that is a bit strange and not the usual semantics for timestamps :-(
>>>>
>>>> Indeed.
>>>>
>>>>>
>>>>> We can't stat the inode - the code was explicitly trying to cope with the
>>>>> way /dev/kvm can change permissions when udev rules get applied. We would
>>>>> have to compare the user, group, permissions mask and even ACL, or a hash
>>>>> of those.
>>>>
>>>> Well, we can use ctime as suggested and post a patch for kernel to fix
>>>> ctime behaviour. Until the patch is merged our behaviour would be
>>>> suboptimal, but still better than it is now.
>>>
>>> I guess lets talk to KVM team for their input on this and then decide
>>> what todo.
>>
>> Hmm, I wonder if it is not actually a kernel problem, but rather something
>> in userspace genuinely touching the device in a way that caues these
>> timestamps to be updated.
>>
>> eg I vaguely recall a udev rule that resets permissions on device nodes
>> whenever an FD is closed, which might cause this kind of behaviour
>
> After trying hard to find the rule that mangles the ctime I've found the
> following bug in udev:
>
> https://github.com/zippy2/systemd/commit/51915e4e6a64c581da321c25448d80626d8e8408?diff=unified
>
> I've send that as a pull request to systemd. So after all, ctime might
> be usable again.

Are there any other objections against this patch? Because even with the
ctime bug in udev it’s performs _much_ better than before.

>
> Michal
>
--
Kind regards / Beste Grüße
   Marc Hartmayer

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