linux-kernel - Re: [PATCH 1/1] mm: introduce process

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Date:   Wed, 30 Jun 2021 17:45:49 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Christoph Hellwig <hch@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/1] mm: introduce process_reap system call

On Wed, Jun 30, 2021 at 11:51 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Wed, Jun 30, 2021 at 11:26 AM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > On Wed, Jun 23, 2021 at 12:28 PM Suren Baghdasaryan <surenb@...gle.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.
> >
> > At the risk of asking a potentially silly question, should this just
> > be a file in procfs?
>
> Hmm. I guess it's doable if procfs will not disappear too soon before
> memory is released... syscall also supports parameters, in this case
> flags can be used in the future to support PIDs in addition to PIDFDs
> for example.
> Before looking more in that direction, a silly question from my side:
> why procfs interface would be preferable to a syscall?

It avoids using a syscall nr.  (Admittedly a syscall nr is not *that*
precious of a resource.)  It also makes it possible to use a shell
script to do this, which is maybe useful.

--Andy