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Date: Fri, 2 Jun 2017 10:09:59 +0800 From: "Yan, Zheng" <ukernel@...il.com> To: Deepa Dinamani <deepa.kernel@...il.com> Cc: John Stultz <john.stultz@...aro.org>, Arnd Bergmann <arnd@...db.de>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, Al Viro <viro@...iv.linux.org.uk>, gregkh <gregkh@...uxfoundation.org>, "Dilger, Andreas" <andreas.dilger@...el.com>, "J. Bruce Fields" <bfields@...ldses.org>, Chris Mason <clm@...com>, David Miller <davem@...emloft.net>, David Sterba <dsterba@...e.com>, Evgeniy Dushistov <dushistov@...l.ru>, Eric Paris <eparis@...hat.com>, Jaegeuk Kim <jaegeuk@...nel.org>, Josef Bacik <jbacik@...com>, Jeff Layton <jlayton@...chiereds.net>, James Simmons <jsimmons@...radead.org>, Ingo Molnar <mingo@...hat.com>, "Drokin, Oleg" <oleg.drokin@...el.com>, Paul Moore <paul@...l-moore.com>, Steven Rostedt <rostedt@...dmis.org>, yuchao0@...wei.com, ceph-devel <ceph-devel@...r.kernel.org>, devel@...verdev.osuosl.org, linux-audit@...hat.com, linux-btrfs <linux-btrfs@...r.kernel.org>, linux-cifs@...r.kernel.org, "Linux F2FS DEV, Mailing List" <linux-f2fs-devel@...ts.sourceforge.net>, Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>, linux-mtd <linux-mtd@...ts.infradead.org>, LSM List <linux-security-module@...r.kernel.org>, Lustre Development List <lustre-devel@...ts.lustre.org>, Networking <netdev@...r.kernel.org>, samba-technical@...ts.samba.org, y2038 Mailman List <y2038@...ts.linaro.org> Subject: Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts() On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@...il.com> wrote: > On Thu, Jun 1, 2017 at 5:36 PM, John Stultz <john.stultz@...aro.org> wrote: >> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel@...il.com> wrote: >>> On Thu, Jun 1, 2017 at 6:22 PM, Arnd Bergmann <arnd@...db.de> wrote: >>>> On Thu, Jun 1, 2017 at 11:56 AM, Yan, Zheng <ukernel@...il.com> wrote: >>>>> On Sat, Apr 8, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@...il.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
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