Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190411014353.113252-1-surenb@google.com> <20190411014353.113252-3-surenb@google.com>
 <20190411153313.GE22763@bombadil.infradead.org> <20190412065314.GC13373@dhcp22.suse.cz>
In-Reply-To: <20190412065314.GC13373@dhcp22.suse.cz>
From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 12 Apr 2019 07:10:08 -0700
Message-ID: <CAJuCfpHzfci6ztBA_c5=DUnfaLQJTphj1UfckN_LZ7w9qi4kAw@mail.gmail.com>
Subject: Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited
 process killing
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        yuzhoujian@didichuxing.com,
        Souptick Joarder <jrdr.linux@gmail.com>,
        Roman Gushchin <guro@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Shakeel Butt <shakeelb@google.com>,
        Christian Brauner <christian@brauner.io>,
        Minchan Kim <minchan@kernel.org>,
        Tim Murray <timmurray@google.com>,
        Daniel Colascione <dancol@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Jann Horn <jannh@google.com>, linux-mm <linux-mm@kvack.org>,
        lsf-pc@lists.linux-foundation.org,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Apr 11, 2019 at 11:53 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Thu 11-04-19 08:33:13, Matthew Wilcox wrote:
> > On Wed, Apr 10, 2019 at 06:43:53PM -0700, Suren Baghdasaryan wrote:
> > > Add new SS_EXPEDITE flag to be used when sending SIGKILL via
> > > pidfd_send_signal() syscall to allow expedited memory reclaim of the
> > > victim process. The usage of this flag is currently limited to SIGKIL=
L
> > > signal and only to privileged users.
> >
> > What is the downside of doing expedited memory reclaim?  ie why not do =
it
> > every time a process is going to die?
>
> Well, you are tearing down an address space which might be still in use
> because the task not fully dead yeat. So there are two downsides AFAICS.
> Core dumping which will not see the reaped memory so the resulting
> coredump might be incomplete. And unexpected #PF/gup on the reaped
> memory will result in SIGBUS. These are things that we have closed our
> eyes in the oom context because they likely do not matter. If we want to
> use the same technique for other usecases then we have to think how much
> that matter again.
>

Sorry, resending with corrected settings...
After some internal discussions we realized that there is one
additional downside of doing reaping unconditionally. If this async
reaping is done on a more efficient and energy hungrier CPU
irrespective of urgency of the kill it should end up costing more
power overall (I=E2=80=99m referring here to assimetric architectures like =
ARM
big.LITTLE). Obviously quantifying that cost is not easy as it depends
on the usecase and a particular system but it won=E2=80=99t be zero. So I
think we will need some gating condition after all.

> --
> Michal Hocko
> SUSE Labs
>
> --
> You received this message because you are subscribed to the Google Groups=
 "kernel-team" group.
> To unsubscribe from this group and stop receiving emails from it, send an=
 email to kernel-team+unsubscribe@android.com.
>