Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

["Yan, Zheng" <ukernel@gmail.com>]

On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
> On Thu, Jun 1, 2017 at 5:36 PM, John Stultz <john.stultz@linaro.org> wrote:
>> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel@gmail.com> wrote:
>>> On Thu, Jun 1, 2017 at 6:22 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>>>> On Thu, Jun 1, 2017 at 11:56 AM, Yan, Zheng <ukernel@gmail.com> wrote:
>>>>> On Sat, Apr 8, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
>>>>
>>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>>> index 517838b..77204da 100644
>>>>>> --- a/drivers/block/rbd.c
>>>>>> +++ b/drivers/block/rbd.c
>>>>>> @@ -1922,7 +1922,7 @@ static void rbd_osd_req_format_write(struct rbd_obj_request *obj_request)
>>>>>>  {
>>>>>>         struct ceph_osd_request *osd_req = obj_request->osd_req;
>>>>>>
>>>>>> -       osd_req->r_mtime = CURRENT_TIME;
>>>>>> +       ktime_get_real_ts(&osd_req->r_mtime);
>>>>>>         osd_req->r_data_offset = obj_request->offset;
>>>>>>  }
>>>>>>
>>>>>> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
>>>>>> index c681762..1d3fa90 100644
>>>>>> --- a/fs/ceph/mds_client.c
>>>>>> +++ b/fs/ceph/mds_client.c
>>>>>> @@ -1666,6 +1666,7 @@ struct ceph_mds_request *
>>>>>>  ceph_mdsc_create_request(struct ceph_mds_client *mdsc, int op, int mode)
>>>>>>  {
>>>>>>         struct ceph_mds_request *req = kzalloc(sizeof(*req), GFP_NOFS);
>>>>>> +       struct timespec ts;
>>>>>>
>>>>>>         if (!req)
>>>>>>                 return ERR_PTR(-ENOMEM);
>>>>>> @@ -1684,7 +1685,8 @@ ceph_mdsc_create_request(struct ceph_mds_client *mdsc, int op, int mode)
>>>>>>         init_completion(&req->r_safe_completion);
>>>>>>         INIT_LIST_HEAD(&req->r_unsafe_item);
>>>>>>
>>>>>> -       req->r_stamp = current_fs_time(mdsc->fsc->sb);
>>>>>> +       ktime_get_real_ts(&ts);
>>>>>> +       req->r_stamp = timespec_trunc(ts, mdsc->fsc->sb->s_time_gran);
>>>>>
>>>>> This change causes our kernel_untar_tar test case to fail (inode's
>>>>> ctime goes back). The reason is that there is time drift between the
>>>>> time stamps got by  ktime_get_real_ts() and current_time(). We need to
>>>>> revert this change until current_time() uses ktime_get_real_ts()
>>>>> internally.
>>>>
>>>> Hmm, the change was not supposed to have a user-visible effect, so
>>>> something has gone wrong, but I don't immediately see how it
>>>> relates to what you observe.
>>>>
>>>> ktime_get_real_ts() and current_time() use the same time base, there
>>>> is no drift, but there is a difference in resolution, as the latter uses
>>>> the time stamp of the last jiffies update, which may be up to one jiffy
>>>> (10ms) behind the exact time we put in the request stamps here.
>>>>
>>>> Do you still see problems if you use current_kernel_time() instead of
>>>> ktime_get_real_ts()?
>>>
>>> The problem disappears after using current_kernel_time().
>>>
>>> https://github.com/ceph/ceph-client/commit/2e0f648da23167034a3cf1500bc90ec60aef2417
>>
>> From the commit above:
>> "It seems there is time drift between ktime_get_real_ts() and
>> current_kernel_time()"
>>
>> Its more of a granularity difference. current_kernel_time() returns
>> the cached time at the last tick, where as ktime_get_real_ts() reads
>> the clocksource hardware and returns the immediate time.
>>
>> Filesystems usually use the cached time (similar to
>> CLOCK_REALTIME_COARSE), for performance reasons, as touching the
>> clocksource takes time.
>
> Alternatively, it would be best for this code also to use current_time().
> I had suggested this in one of the previous versions of the patch.
> The implementation of current_time() will change when we switch vfs to
> use 64 bit time. This will prevent such errors from happening again.
> But, this also means there is more code reordering for these modules
> to get a reference to inode.
>

I took a look. it's quite inconvenience to use current_time(). I
prefer to temporarily use current_kernel_time().

Regards
Yan, Zheng



> -Deepa