18c91bb2d8
Unnecessarily grabbing the tasklist_lock can be a scalability bottleneck for workloads that also must grab the tasklist_lock for waiting, killing, and cloning. The tasklist_lock was grabbed to protect tsk->sighand from disappearing (becoming NULL). tsk->signal was already protected by holding a reference to tsk. update_rlimit_cpu() assumed tsk->sighand != NULL. With this commit, it attempts to lock_task_sighand(). However, this means that update_rlimit_cpu() can fail. This only happens when a task is exiting. Note that during exec, sighand may *change*, but it will not be NULL. Prior to this commit, the do_prlimit() ensured that update_rlimit_cpu() would not fail by read locking the tasklist_lock and checking tsk->sighand != NULL. If update_rlimit_cpu() fails, there may be other tasks that are not exiting that share tsk->signal. However, the group_leader is the last task to be released, so if we cannot update_rlimit_cpu(group_leader), then the entire process is exiting. The only other caller of update_rlimit_cpu() is selinux_bprm_committing_creds(). It has tsk == current, so update_rlimit_cpu() cannot fail (current->sighand cannot disappear until current exits). This change resulted in a 14% speedup on a microbenchmark where parents kill and wait on their children, and children getpriority, setpriority, and getrlimit. Signed-off-by: Barret Rhoden <brho@google.com> Link: https://lkml.kernel.org/r/20220106172041.522167-4-brho@google.com Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
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.