From 18c91bb2d87268d23868bf13508f5bc9cf04e89a Mon Sep 17 00:00:00 2001 From: Barret Rhoden Date: Thu, 6 Jan 2022 12:20:41 -0500 Subject: prlimit: do not grab the tasklist_lock 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 Link: https://lkml.kernel.org/r/20220106172041.522167-4-brho@google.com Signed-off-by: Eric W. Biederman --- include/linux/posix-timers.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux') diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h index 5bbcd280bfd2..9cf126c3b27f 100644 --- a/include/linux/posix-timers.h +++ b/include/linux/posix-timers.h @@ -253,7 +253,7 @@ void posix_cpu_timers_exit_group(struct task_struct *task); void set_process_cpu_timer(struct task_struct *task, unsigned int clock_idx, u64 *newval, u64 *oldval); -void update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new); +int update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new); void posixtimer_rearm(struct kernel_siginfo *info); #endif -- cgit v1.2.3