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
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement
Date: Tue, 15 Dec 2020 20:59:17 +0800 [thread overview]
Message-ID: <efb7469c7bad2f6458c9a537b8e3623e7c303c21.camel@themaw.net> (raw)
In-Reply-To: <CAC2o3DLGtx15cgra3Y92UBdQRBKGckqOkDmwBV-aV-EpUqO5SQ@mail.gmail.com>
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
next prev parent reply other threads:[~2020-12-15 13:00 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-17 7:37 [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement Ian Kent
2020-06-17 7:37 ` [PATCH v2 1/6] kernfs: switch kernfs to use an rwsem Ian Kent
2020-06-17 7:37 ` [PATCH v2 2/6] kernfs: move revalidate to be near lookup Ian Kent
2020-06-17 7:37 ` [PATCH v2 3/6] kernfs: improve kernfs path resolution Ian Kent
2020-06-17 7:38 ` [PATCH v2 4/6] kernfs: use revision to identify directory node changes Ian Kent
2020-06-17 7:38 ` [PATCH v2 5/6] kernfs: refactor attr locking Ian Kent
2020-06-17 7:38 ` [PATCH v2 6/6] kernfs: make attr_mutex a local kernfs node lock Ian Kent
2020-06-19 15:38 ` [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement Tejun Heo
2020-06-19 20:41 ` Rick Lindsley
2020-06-19 22:23 ` Tejun Heo
2020-06-20 2:44 ` Rick Lindsley
2020-06-22 17:53 ` Tejun Heo
2020-06-22 21:22 ` Rick Lindsley
2020-06-23 23:13 ` Tejun Heo
2020-06-24 9:04 ` Rick Lindsley
2020-06-24 9:27 ` Greg Kroah-Hartman
2020-06-24 13:19 ` Tejun Heo
2020-06-25 8:15 ` Ian Kent
2020-06-25 9:43 ` Greg Kroah-Hartman
2020-06-26 0:19 ` Ian Kent
2020-06-21 4:55 ` Ian Kent
2020-06-22 17:48 ` Tejun Heo
2020-06-22 18:03 ` Greg Kroah-Hartman
2020-06-22 21:27 ` Rick Lindsley
2020-06-23 5:21 ` Greg Kroah-Hartman
2020-06-23 5:09 ` Ian Kent
2020-06-23 6:02 ` Greg Kroah-Hartman
2020-06-23 8:01 ` Ian Kent
2020-06-23 8:29 ` Ian Kent
2020-06-23 11:49 ` Greg Kroah-Hartman
2020-06-23 9:33 ` Rick Lindsley
2020-06-23 11:45 ` Greg Kroah-Hartman
2020-06-23 22:55 ` Rick Lindsley
2020-06-23 11:51 ` Ian Kent
2020-06-21 3:21 ` Ian Kent
2020-12-10 16:44 ` Fox Chen
2020-12-11 2:01 ` [PATCH " Ian Kent
2020-12-11 2:17 ` Ian Kent
2020-12-13 3:46 ` Ian Kent
2020-12-14 6:14 ` Fox Chen
2020-12-14 13:30 ` Ian Kent
2020-12-15 8:33 ` Fox Chen
2020-12-15 12:59 ` Ian Kent [this message]
2020-12-17 4:46 ` Ian Kent
2020-12-17 8:54 ` Fox Chen
2020-12-17 10:09 ` Ian Kent
2020-12-17 11:09 ` Ian Kent
2020-12-17 11:48 ` Ian Kent
2020-12-17 15:14 ` Tejun Heo
2020-12-18 7:36 ` Ian Kent
2020-12-18 8:01 ` Fox Chen
2020-12-18 11:21 ` Ian Kent
2020-12-18 13:20 ` Fox Chen
2020-12-19 0:53 ` Ian Kent
2020-12-19 7:47 ` Fox Chen
2020-12-22 2:17 ` Ian Kent
2020-12-18 14:59 ` Tejun Heo
2020-12-19 7:08 ` Ian Kent
2020-12-19 16:23 ` Tejun Heo
2020-12-19 23:52 ` Ian Kent
2020-12-20 1:37 ` Ian Kent
2020-12-21 9:28 ` Fox Chen
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