Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751195AbdFBCKF (ORCPT ); Thu, 1 Jun 2017 22:10:05 -0400 Received: from mail-vk0-f67.google.com ([209.85.213.67]:32920 "EHLO mail-vk0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751120AbdFBCKB (ORCPT ); Thu, 1 Jun 2017 22:10:01 -0400 MIME-Version: 1.0 In-Reply-To: References: <1491613030-11599-1-git-send-email-deepa.kernel@gmail.com> <1491613030-11599-5-git-send-email-deepa.kernel@gmail.com> From: "Yan, Zheng" Date: Fri, 2 Jun 2017 10:09:59 +0800 Message-ID: Subject: Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts() To: Deepa Dinamani Cc: John Stultz , Arnd Bergmann , Linux Kernel Mailing List , Andrew Morton , Thomas Gleixner , Al Viro , gregkh , "Dilger, Andreas" , "J. Bruce Fields" , Chris Mason , David Miller , David Sterba , Evgeniy Dushistov , Eric Paris , Jaegeuk Kim , Josef Bacik , Jeff Layton , James Simmons , Ingo Molnar , "Drokin, Oleg" , Paul Moore , Steven Rostedt , yuchao0@huawei.com, ceph-devel , devel@driverdev.osuosl.org, linux-audit@redhat.com, linux-btrfs , linux-cifs@vger.kernel.org, "Linux F2FS DEV, Mailing List" , Linux FS-devel Mailing List , linux-mtd , LSM List , Lustre Development List , Networking , samba-technical@lists.samba.org, y2038 Mailman List Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3996 Lines: 91 On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani wrote: > On Thu, Jun 1, 2017 at 5:36 PM, John Stultz wrote: >> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng wrote: >>> On Thu, Jun 1, 2017 at 6:22 PM, Arnd Bergmann wrote: >>>> On Thu, Jun 1, 2017 at 11:56 AM, Yan, Zheng wrote: >>>>> On Sat, Apr 8, 2017 at 8:57 AM, Deepa Dinamani 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