Date:   Thu, 18 Nov 2021 17:43:49 +0100
From:   Jan Kara <jack@suse.cz>
To:     Chengguang Xu <cgxu519@mykernel.net>
Cc:     Jan Kara <jack@suse.cz>, Miklos Szeredi <miklos@szeredi.hu>,
        Amir Goldstein <amir73il@gmail.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode
 operation
Message-ID: <20211118164349.GB8267@quack2.suse.cz>
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On Thu 18-11-21 20:02:09, Chengguang Xu wrote:
>  ---- 在 星期四, 2021-11-18 19:23:15 Jan Kara <jack@suse.cz> 撰写 ----
>  > On Thu 18-11-21 14:32:36, Chengguang Xu wrote:
>  > > 
>  > >  ---- 在 星期三, 2021-11-17 14:11:29 Chengguang Xu <cgxu519@mykernel.net> 撰写 ----
>  > >  >  ---- 在 星期二, 2021-11-16 20:35:55 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
>  > >  >  > On Tue, 16 Nov 2021 at 03:20, Chengguang Xu <cgxu519@mykernel.net> wrote:
>  > >  >  > >
>  > >  >  > >  ---- 在 星期四, 2021-10-07 21:34:19 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
>  > >  >  > >  > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@mykernel.net> wrote:
>  > >  >  > >  > >  > However that wasn't what I was asking about.  AFAICS ->write_inode()
>  > >  >  > >  > >  > won't start write back for dirty pages.   Maybe I'm missing something,
>  > >  >  > >  > >  > but there it looks as if nothing will actually trigger writeback for
>  > >  >  > >  > >  > dirty pages in upper inode.
>  > >  >  > >  > >  >
>  > >  >  > >  > >
>  > >  >  > >  > > Actually, page writeback on upper inode will be triggered by overlayfs ->writepages and
>  > >  >  > >  > > overlayfs' ->writepages will be called by vfs writeback function (i.e writeback_sb_inodes).
>  > >  >  > >  >
>  > >  >  > >  > Right.
>  > >  >  > >  >
>  > >  >  > >  > But wouldn't it be simpler to do this from ->write_inode()?
>  > >  >  > >  >
>  > >  >  > >  > I.e. call write_inode_now() as suggested by Jan.
>  > >  >  > >  >
>  > >  >  > >  > Also could just call mark_inode_dirty() on the overlay inode
>  > >  >  > >  > regardless of the dirty flags on the upper inode since it shouldn't
>  > >  >  > >  > matter and results in simpler logic.
>  > >  >  > >  >
>  > >  >  > >
>  > >  >  > > Hi Miklos，
>  > >  >  > >
>  > >  >  > > Sorry for delayed response for this, I've been busy with another project.
>  > >  >  > >
>  > >  >  > > I agree with your suggesion above and further more how about just mark overlay inode dirty
>  > >  >  > > when it has upper inode? This approach will make marking dirtiness simple enough.
>  > >  >  > 
>  > >  >  > Are you suggesting that all non-lower overlay inodes should always be dirty?
>  > >  >  > 
>  > >  >  > The logic would be simple, no doubt, but there's the cost to walking
>  > >  >  > those overlay inodes which don't have a dirty upper inode, right?  
>  > >  > 
>  > >  > That's true.
>  > >  > 
>  > >  >  > Can you quantify this cost with a benchmark?  Can be totally synthetic,
>  > >  >  > e.g. lookup a million upper files without modifying them, then call
>  > >  >  > syncfs.
>  > >  >  > 
>  > >  > 
>  > >  > No problem, I'll do some tests for the performance.
>  > >  > 
>  > > 
>  > > Hi Miklos,
>  > > 
>  > > I did some rough tests and the results like below.  In practice,  I don't
>  > > think that 1.3s extra time of syncfs will cause significant problem.
>  > > What do you think?
>  > 
>  > Well, burning 1.3s worth of CPU time for doing nothing seems like quite a
>  > bit to me. I understand this is with 1000000 inodes but although that is
>  > quite a few it is not unheard of. If there would be several containers
>  > calling sync_fs(2) on the machine they could easily hog the machine... That
>  > is why I was originally against keeping overlay inodes always dirty and
>  > wanted their dirtiness to at least roughly track the real need to do
>  > writeback.
>  > 
> 
> Hi Jan,
> 
> Actually, the time on user and sys are almost same with directly excute syncfs on underlying fs.
> IMO, it only extends syncfs(2) waiting time for perticular container but not burning cpu.
> What am I missing?

Ah, right, I've missed that only realtime changed, not systime. I'm sorry
for confusion. But why did the realtime increase so much? Are we waiting
for some IO?

								Honza

>  > > Test bed: kvm vm 
>  > > 2.50GHz cpu 32core
>  > > 64GB mem
>  > > vm kernel  5.15.0-rc1+ (with ovl syncfs patch V6)
>  > > 
>  > > one millon files spread to 2 level of dir hierarchy.
>  > > test step:
>  > > 1) create testfiles in ovl upper dir
>  > > 2) mount overlayfs
>  > > 3) excute ls -lR to lookup all file in overlay merge dir
>  > > 4) excute slabtop to make sure overlay inode number
>  > > 5) call syncfs to the file in merge dir
>  > > 
>  > > Tested five times and the reusults are in 1.310s ~ 1.326s
>  > > 
>  > > root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m1.310s
>  > > user    0m0.000s
>  > > sys     0m0.001s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m1.326s
>  > > user    0m0.001s
>  > > sys     0m0.000s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m1.321s
>  > > user    0m0.000s
>  > > sys     0m0.001s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m1.316s
>  > > user    0m0.000s
>  > > sys     0m0.001s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-merge/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m1.314s
>  > > user    0m0.001s
>  > > sys     0m0.001s
>  > > 
>  > > 
>  > > Directly run syncfs to the file in ovl-upper dir.
>  > > Tested five times and the reusults are in 0.001s ~ 0.003s
>  > > 
>  > > [root@VM-144-4-centos test]# time ./syncfs a
>  > > syncfs success
>  > > 
>  > > real    0m0.002s
>  > > user    0m0.001s
>  > > sys     0m0.000s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m0.003s
>  > > user    0m0.001s
>  > > sys     0m0.000s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m0.001s
>  > > user    0m0.000s
>  > > sys     0m0.001s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m0.001s
>  > > user    0m0.000s
>  > > sys     0m0.001s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m0.001s
>  > > user    0m0.000s
>  > > sys     0m0.001s
>  > > [root@VM-144-4-centos test]# time ./syncfs ovl-upper/create-file.sh 
>  > > syncfs success
>  > > 
>  > > real    0m0.001s
>  > > user    0m0.000s
>  > > sys     0m0.001
>  > > 
>  > > 
>  > > 
>  > > 
>  > > 
>  > > 
>  > -- 
>  > Jan Kara <jack@suse.com>
>  > SUSE Labs, CR
>  > 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR