1b6b26ae70
The pipe rework ends up having been extra painful, partly becaused of actual bugs with ordering and caching of the pipe state, but also because of subtle performance issues. In particular, the pipe rework caused the kernel build to inexplicably slow down. The reason turns out to be that the GNU make jobserver (which limits the parallelism of the build) uses a pipe to implement a "token" system: a parallel submake will read a character from the pipe to get the job token before starting a new job, and will write a character back to the pipe when it is done. The overall job limit is thus easily controlled by just writing the appropriate number of initial token characters into the pipe. But to work well, that really means that the old behavior of write wakeups being synchronous (WF_SYNC) is very important - when the pipe writer wakes up a reader, we want the reader to actually get scheduled immediately. Otherwise you lose the parallelism of the build. The pipe rework lost that synchronous wakeup on write, and we had clearly all forgotten the reasons and rules for it. This rewrites the pipe write wakeup logic to do the required Wsync wakeups, but also clarifies the logic and avoids extraneous wakeups. It also ends up addign a number of comments about what oit does and why, so that we hopefully don't end up forgetting about this next time we change this code. Cc: David Howells <dhowells@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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.