27.09.2021

В ядро Linux 5.15 добавлен системный вызов для быстрого освобождения памяти умирающего процесса

Новый системный вызов называется process_mrelease и он позволяет ускорить освобождение памяти, используемой процессом, получившим сигнал SIGKILL. Системный вызов process_mrelease получает два параметра — pid умирающего процесса и flags. В текущей реализации flags не используется и должен быть нулём, но это говорит о том, что в будущем функциональность этого системного вызова может быть расширена. Возвращаемые значения: 0 если всё хорошо и -1 если произошла ошибка, код которой передаётся через errno.

Где будет использоваться этот системный вызов? Видимо в сервисах типа systemd-oomd и lmkd.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=884a7e5964e06ed93c7771c0d7cf19c09a8946f1

In modern systems it's not unusual to have a system component monitoring
memory conditions of the system and tasked with keeping system memory
pressure under control. One way to accomplish that is to kill
non-essential processes to free up memory for more important ones.
Examples of this are Facebook's OOM killer daemon called oomd and
Android's low memory killer daemon called lmkd. For such system component it's important to be able to free memory quickly
and efficiently. Unfortunately the time process takes to free up its
memory after receiving a SIGKILL might vary based on the state of the
process (uninterruptible sleep), size and OPP level of the core the
process is running. A mechanism to free resources of the target process
in a more predictable way would improve system's ability to control its
memory pressure. Introduce process_mrelease system call that releases memory of a dying
process from the context of the caller. This way the memory is freed in a
more controllable way with CPU affinity and priority of the caller. The
workload of freeing the memory will also be charged to the caller. The
operation is allowed only on a dying process. After previous discussions [1, 2, 3] the decision was made [4] to
introduce a dedicated system call to cover this use case. The API is as follows, int process_mrelease(int pidfd, unsigned int flags); DESCRIPTION The process_mrelease() system call is used to free the memory of an exiting process. The pidfd selects the process referred to by the PID file descriptor. (See pidfd_open(2) for further information) The flags argument is reserved for future use; currently, this argument must be specified as 0. RETURN VALUE On success, process_mrelease() returns 0. On error, -1 is returned and errno is set to indicate the error. ERRORS EBADF pidfd is not a valid PID file descriptor. EAGAIN Failed to release part of the address space. EINTR The call was interrupted by a signal; see signal(7). EINVAL flags is not 0. EINVAL The memory of the task cannot be released because the process is not exiting, the address space is shared with another live process or there is a core dump in progress. ENOSYS This system call is not supported, for example, without MMU support built into Linux. ESRCH The target process does not exist (i.e., it has terminated and been waited on). [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
[2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
[3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
[4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/

>>> Подробности

Источник.