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dba2e3b788Mateusz Guzik <mjguzik@gmail.com> says:
The stock kernel transitioning the file to no refs held penalizes the
caller with an extra atomic to block any increments.
For cases where the file is highly likely to be going away this is
easily avoidable.
In the open+close case the win is very modest because of the following
problems:
- kmem and memcg having terrible performance
- putname using an atomic (I have a wip to whack that)
- open performing an extra ref/unref on the dentry (there are patches to
do it, including by Al. I mailed about them in [1])
- creds using atomics (I have a wip to whack that)
- apparmor using atomics (ditto, same mechanism)
On top of that I have a WIP patch to dodge some of the work at lookup
itself.
All in all there is several % avoidably lost here.
stats colected during a kernel build with:
bpftrace -e 'kprobe:filp_close,kprobe:fput,kprobe:fput_close* { @[probe] = hist(((struct file *)arg0)->f_ref.refcnt.counter > 0); }'
@[kprobe:filp_close]:
[0] 32195 |@@@@@@@@@@ |
[1] 164567 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
@[kprobe:fput]:
[0] 339240 |@@@@@@ |
[1] 2888064 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
@[kprobe:fput_close]:
[0]
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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 reStructuredText 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.