From: Greg Kroah-Hartman <firstname.lastname@example.org> To: Minchan Kim <email@example.com> Cc: Tejun Heo <firstname.lastname@example.org>, LKML <email@example.com> Subject: Re: [RFC PATCH] kernfs: release kernfs_mutex before the inode allocation Date: Wed, 17 Nov 2021 08:39:44 +0100 [thread overview] Message-ID: <YZSxwM8ucqGsY1hq@kroah.com> (raw) In-Reply-To: <YZSu/HiHDZxo9Wpa@google.com> On Tue, Nov 16, 2021 at 11:27:56PM -0800, Minchan Kim wrote: > On Wed, Nov 17, 2021 at 07:44:44AM +0100, Greg Kroah-Hartman wrote: > > On Tue, Nov 16, 2021 at 01:36:01PM -0800, Minchan Kim wrote: > > > On Tue, Nov 16, 2021 at 08:49:46PM +0100, Greg Kroah-Hartman wrote: > > > > On Tue, Nov 16, 2021 at 11:43:17AM -0800, Minchan Kim wrote: > > > > > The kernfs implementation has big lock granularity(kernfs_rwsem) so > > > > > every kernfs-based(e.g., sysfs, cgroup, dmabuf) fs are able to compete > > > > > the lock. Thus, if one of userspace goes the sleep under holding > > > > > the lock for a long time, rest of them should wait it. A example is > > > > > the holder goes direct reclaim with the lock since it needs memory > > > > > allocation. Let's fix it at common technique that release the lock > > > > > and then allocate the memory. Fortunately, kernfs looks like have > > > > > an refcount so I hope it's fine. > > > > > > > > > > Signed-off-by: Minchan Kim <firstname.lastname@example.org> > > > > > --- > > > > > fs/kernfs/dir.c | 14 +++++++++++--- > > > > > fs/kernfs/inode.c | 2 +- > > > > > fs/kernfs/kernfs-internal.h | 1 + > > > > > 3 files changed, 13 insertions(+), 4 deletions(-) > > > > > > > > What workload hits this lock to cause it to be noticable? > > > > > > A app launching since it was dropping the frame since the > > > latency was too long. > > > > How does running a program interact with kernfs filesystems? Which > > one(s)? > > A app launching involves dma_buf exports which creates kobject > and add it to the kernfs with down_write - kernfs_add_one. I thought the "create a dma_buf kobject" kernel change was fixed up to not do that anymore as that was a known performance issue. Creating kobjects should NOT be on a fast path, if they are, that needs to be fixed. > At the same time in other CPU, a random process was accessing > sysfs and the kernfs_iop_lookup was already hoding the kernfs_rwsem > and ran under direct reclaim patch due to alloc_inode in > kerfs_get_inode. What program is constantly hitting sysfs? sysfs is not for performance-critical things, right? > Therefore, the app is stuck on the lock and lose frames so enduser > sees the jank. But how does this patch work around it? It seems like you are special-casing the kobject creation path only. And is this the case really on 5.15? I thought the kernfs locks were broken up again to not cause this problem in 5.14 or so. thanks, greg k-h
next prev parent reply other threads:[~2021-11-17 7:39 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-11-16 19:43 Minchan Kim 2021-11-16 19:49 ` Greg Kroah-Hartman 2021-11-16 21:36 ` Minchan Kim 2021-11-17 6:44 ` Greg Kroah-Hartman 2021-11-17 7:27 ` Minchan Kim 2021-11-17 7:39 ` Greg Kroah-Hartman [this message] 2021-11-17 21:43 ` Minchan Kim 2021-11-17 21:45 ` Tejun Heo 2021-11-17 22:13 ` Minchan Kim 2021-11-17 22:23 ` Tejun Heo 2021-11-18 1:55 ` Minchan Kim 2021-11-18 16:35 ` Tejun Heo
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