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Mainline Linux tree for various devices, only for fun :)
e9c15badbb
The kernel uses internal mounts created by kern_mount() and populated
with files with no lookup path by alloc_file_pseudo() for a variety of
reasons. An example of such a mount is for anonymous pipes. For pipes,
every vfs_write() regardless of filesystem, calls fsnotify_modify()
to notify of any changes which incurs a small amount of overhead in
fsnotify even when there are no watchers. It can also trigger for reads
and readv and writev, it was simply vfs_write() that was noticed first.
A patch is pending that reduces, but does not eliminate, the overhead of
fsnotify but for files that cannot be looked up via a path, even that
small overhead is unnecessary. The user API for all notification
subsystems (inotify, fanotify, ...) is based on the pathname and a dirfd
and proc entries appear to be the only visible representation of the
files. Proc does not have the same pathname as the internal entry and
the proc inode is not the same as the internal inode so even if fanotify
is used on a file under /proc/XX/fd, no useful events are notified.
This patch changes alloc_file_pseudo() to always opt out of fsnotify by
setting FMODE_NONOTIFY flag so that no check is made for fsnotify
watchers on pseudo files. This should be safe as the underlying helper
for the dentry is d_alloc_pseudo() which explicitly states that no
lookups are ever performed meaning that fanotify should have nothing
useful to attach to.
The test motivating this was "perf bench sched messaging --pipe". On
a single-socket machine using threads the difference of the patch was
as follows.
5.7.0 5.7.0
vanilla nofsnotify-v1r1
Amean 1 1.3837 ( 0.00%) 1.3547 ( 2.10%)
Amean 3 3.7360 ( 0.00%) 3.6543 ( 2.19%)
Amean 5 5.8130 ( 0.00%) 5.7233 * 1.54%*
Amean 7 8.1490 ( 0.00%) 7.9730 * 2.16%*
Amean 12 14.6843 ( 0.00%) 14.1820 ( 3.42%)
Amean 18 21.8840 ( 0.00%) 21.7460 ( 0.63%)
Amean 24 28.8697 ( 0.00%) 29.1680 ( -1.03%)
Amean 30 36.0787 ( 0.00%) 35.2640 * 2.26%*
Amean 32 38.0527 ( 0.00%) 38.1223 ( -0.18%)
The difference is small but in some cases it's outside the noise so
while marginal, there is still some small benefit to ignoring fsnotify
for files allocated via alloc_file_pseudo() in some cases.
Link: https://lore.kernel.org/r/20200615121358.GF3183@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.