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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 30 Jun 2021 17:45:49 -0700
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Subject: Re: [PATCH 1/1] mm: introduce process_reap system call
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
        Minchan Kim <minchan@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Christoph Hellwig <hch@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Jann Horn <jannh@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Tim Murray <timmurray@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>
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On Wed, Jun 30, 2021 at 11:51 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Wed, Jun 30, 2021 at 11:26 AM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > 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.
> >
> > 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