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

On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel(a)gmail.com&gt; wrote: > On Thu, Jun 1, 2017 at 6:22 PM, Arnd Bergmann <arnd(a)arndb.de&gt; wrote: >> On Thu, Jun 1, 2017 at 11:56 AM, Yan, Zheng <ukernel(a)gmail.com&gt; wrote: >>> On Sat, Apr 8, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel(a)gmail.com&gt; 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/2e0f648da23167034a3cf1500bc90e... 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.