Re: [PATCH 1/1] mm: introduce process_reap system call

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jun 30, 2021 at 11:01 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> 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

Ack

>
> >
> >           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?

Yes, at least that's how Android does that. It's a bit more involved
but it's a technical detail. Userspace low memory killer kills a
process (sends SIGKILL and calls process_reap) and another system
component detects that a process died and will kill all processes
belonging to the same cgroup (that's how we identify related
processes).

>
> 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).

Seems reasonable to me. We could use that in the above scenario.

>
> [...]
>
> > +
> > +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.

Hmm. But I think we still need to make sure that the mm is not shared
with another non-dying process. IIUC that's the point of that
traversal. Am I mistaken?