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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 1 Jul 2021 16:08:01 -0700
Message-ID: <CAJuCfpEWmcJSKuAD+4yOcWyTTDTo0p7Aico5FruXZcyZt120Jg@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm: introduce process_reap system call
To:     Andy Lutomirski <luto@kernel.org>
Cc:     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 5:46 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> 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.

I see. Not really sure if the shell usage is a big usecase for this
operation but let's see if more people like that approach. For my
specific usecase one syscall (process_reap) is better than three
syscalls (open, write, close) and the possibility to extend the
functionality using flags might be of value for the future.

>
> --Andy