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    SubjectRe: [PATCH 1/1] mm: introduce process_reap system call
    Hi Suren,

    On Wed, Jun 23, 2021 at 12:28 PM Suren Baghdasaryan <surenb@google.com> wrote:
    >
    > 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_reap system call that reclaims memory of a dying process
    > from the context of the caller. This way the memory in 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.
    >
    > Previously I proposed a number of alternatives to accomplish this:
    > - https://lore.kernel.org/patchwork/patch/1060407 extending
    > pidfd_send_signal to allow memory reaping using oom_reaper thread;
    > - https://lore.kernel.org/patchwork/patch/1338196 extending
    > pidfd_send_signal to reap memory of the target process synchronously from
    > the context of the caller;
    > - https://lore.kernel.org/patchwork/patch/1344419/ to add MADV_DONTNEED
    > support for process_madvise implementing synchronous memory reaping.
    >
    > The end of the last discussion culminated with suggestion to introduce a
    > dedicated system call (https://lore.kernel.org/patchwork/patch/1344418/#1553875)
    > The reasoning was that the new variant of process_madvise
    > a) does not work on an address range
    > b) is destructive
    > c) doesn't share much code at all with the rest of process_madvise
    > From the userspace point of view it was awkward and inconvenient to provide
    > memory range for this operation that operates on the entire address space.
    > Using special flags or address values to specify the entire address space
    > was too hacky.
    >
    > The API is as follows,
    >
    > int process_reap(int pidfd, unsigned int flags);
    >
    > DESCRIPTION
    > The process_reap() system call is used to free the memory of a
    > dying process.
    >
    > The pidfd selects the process referred to by the PID file
    > descriptor.
    > (See pidofd_open(2) for further information)

    *pidfd_open

    >
    > The flags argument is reserved for future use; currently, this
    > argument must be specified as 0.
    >
    > RETURN VALUE
    > On success, process_reap() returns 0. On error, -1 is returned
    > and errno is set to indicate the error.
    >
    > Signed-off-by: Suren Baghdasaryan <surenb@google.com>

    Thanks for continuously pushing this. One question I have is how do
    you envision this syscall to be used for the cgroup based workloads.
    Traverse the target tree, read pids from cgroup.procs files,
    pidfd_open them, send SIGKILL and then process_reap them. Is that
    right?

    Orthogonal to this patch I wonder if we should have an optimized way
    to reap processes from a cgroup. Something similar to cgroup.kill (or
    maybe overload cgroup.kill with reaping as well).

    [...]

    > +
    > +SYSCALL_DEFINE2(process_reap, int, pidfd, unsigned int, flags)
    > +{
    > + struct pid *pid;
    > + struct task_struct *task;
    > + struct mm_struct *mm = NULL;
    > + unsigned int f_flags;
    > + long ret = 0;
    > +
    > + if (flags != 0)
    > + return -EINVAL;
    > +
    > + pid = pidfd_get_pid(pidfd, &f_flags);
    > + if (IS_ERR(pid))
    > + return PTR_ERR(pid);
    > +
    > + task = get_pid_task(pid, PIDTYPE_PID);
    > + if (!task) {
    > + ret = -ESRCH;
    > + goto put_pid;
    > + }
    > +
    > + /*
    > + * If the task is dying and in the process of releasing its memory
    > + * then get its mm.
    > + */
    > + task_lock(task);
    > + if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {

    task_will_free_mem() is fine here but I think in parallel we should
    optimize this function. At the moment it is traversing all the
    processes on the machine. It is very normal to have tens of thousands
    of processes on big machines, so it would be really costly when
    reaping a bunch of processes.

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