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Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement
From:   Ian Kent <raven@themaw.net>
To:     Fox Chen <foxhlchen@gmail.com>
Cc:     akpm@linux-foundation.org, dhowells@redhat.com,
        Greg KH <gregkh@linuxfoundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        miklos@szeredi.hu, ricklind@linux.vnet.ibm.com,
        sfr@canb.auug.org.au, Tejun Heo <tj@kernel.org>,
        viro@zeniv.linux.org.uk
Date:   Tue, 15 Dec 2020 20:59:17 +0800
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On Tue, 2020-12-15 at 16:33 +0800, Fox Chen wrote:
> On Mon, Dec 14, 2020 at 9:30 PM Ian Kent <raven@themaw.net> wrote:
> > On Mon, 2020-12-14 at 14:14 +0800, Fox Chen wrote:
> > > On Sun, Dec 13, 2020 at 11:46 AM Ian Kent <raven@themaw.net>
> > > wrote:
> > > > On Fri, 2020-12-11 at 10:17 +0800, Ian Kent wrote:
> > > > > On Fri, 2020-12-11 at 10:01 +0800, Ian Kent wrote:
> > > > > > > For the patches, there is a mutex_lock in kn->attr_mutex, 
> > > > > > > as
> > > > > > > Tejun
> > > > > > > mentioned here
> > > > > > > (
> > > > > > > https://lore.kernel.org/lkml/X8fe0cmu+aq1gi7O@mtj.duckdns.org/
> > > > > > > ),
> > > > > > > maybe a global
> > > > > > > rwsem for kn->iattr will be better??
> > > > > > 
> > > > > > I wasn't sure about that, IIRC a spin lock could be used
> > > > > > around
> > > > > > the
> > > > > > initial check and checked again at the end which would
> > > > > > probably
> > > > > > have
> > > > > > been much faster but much less conservative and a bit more
> > > > > > ugly
> > > > > > so
> > > > > > I just went the conservative path since there was so much
> > > > > > change
> > > > > > already.
> > > > > 
> > > > > Sorry, I hadn't looked at Tejun's reply yet and TBH didn't
> > > > > remember
> > > > > it.
> > > > > 
> > > > > Based on what Tejun said it sounds like that needs work.
> > > > 
> > > > Those attribute handling patches were meant to allow taking the
> > > > rw
> > > > sem read lock instead of the write lock for
> > > > kernfs_refresh_inode()
> > > > updates, with the added locking to protect the inode attributes
> > > > update since it's called from the VFS both with and without the
> > > > inode lock.
> > > 
> > > Oh, understood. I was asking also because lock on kn->attr_mutex
> > > drags
> > > concurrent performance.
> > > 
> > > > Looking around it looks like kernfs_iattrs() is called from
> > > > multiple
> > > > places without a node database lock at all.
> > > > 
> > > > I'm thinking that, to keep my proposed change straight forward
> > > > and on topic, I should just leave kernfs_refresh_inode() taking
> > > > the node db write lock for now and consider the attributes
> > > > handling
> > > > as a separate change. Once that's done we could reconsider
> > > > what's
> > > > needed to use the node db read lock in kernfs_refresh_inode().
> > > 
> > > You meant taking write lock of kernfs_rwsem for
> > > kernfs_refresh_inode()??
> > > It may be a lot slower in my benchmark, let me test it.
> > 
> > Yes, but make sure the write lock of kernfs_rwsem is being taken
> > not the read lock.
> > 
> > That's a mistake I had initially?
> > 
> > Still, that attributes handling is, I think, sufficient to warrant
> > a separate change since it looks like it might need work, the
> > kernfs
> > node db probably should be kept stable for those attribute updates
> > but equally the existence of an instantiated dentry might mitigate
> > the it.
> > 
> > Some people might just know whether it's ok or not but I would like
> > to check the callers to work out what's going on.
> > 
> > In any case it's academic if GCH isn't willing to consider the
> > series
> > for review and possible merge.
> > 
> Hi Ian
> 
> I removed kn->attr_mutex and changed read lock to write lock for
> kernfs_refresh_inode
> 
> down_write(&kernfs_rwsem);
> kernfs_refresh_inode(kn, inode);
> up_write(&kernfs_rwsem);
> 
> 
> Unfortunate, changes in this way make things worse,  my benchmark
> runs
> 100% slower than upstream sysfs.  :(
> open+read+close a sysfs file concurrently took 1000us. (Currently,
> sysfs with a big mutex kernfs_mutex only takes ~500us
> for one open+read+close operation concurrently)

Right, so it does need attention nowish.

I'll have a look at it in a while, I really need to get a new autofs
release out, and there are quite a few changes, and testing is seeing
a number of errors, some old, some newly introduced. It's proving
difficult.

> 
> > --45.93%--kernfs_iop_permission
>                                   |                                |
>                   |          |          |          |
>                                   |                                |
>                   |          |          |
> > --22.55%--down_write
>                                   |                                |
>                   |          |          |          |          |
>                                   |                                |
>                   |          |          |          |
> --20.69%--rwsem_down_write_slowpath
>                                   |                                |
>                   |          |          |          |
>   |
>                                   |                                |
>                   |          |          |          |
>   |--8.89%--schedule
> 
> perf showed most of the time had been spent on kernfs_iop_permission
> 
> 
> thanks,
> fox