linux-kernel - Re: [RFC v3 5/8] dmabuf: Add gpu cgroup charge transfer function

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Message-ID: <CABdmKX2hZChBO09xfhqB7EbH6RY9JdmDp7zh23DaGuwidn=v4w@mail.gmail.com>
Date:   Tue, 22 Mar 2022 09:47:59 -0700
From:   "T.J. Mercier" <tjmercier@...gle.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
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        Sumit Semwal <sumit.semwal@...aro.org>,
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        Shuah Khan <shuah@...nel.org>,
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        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
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        "Subject: Re: [RFC v3 5/8] dmabuf: Add gpu cgroup charge transfer
        function Reply-To: In-Reply-To:" 
        <CABdmKX3+mTjxWzgrv44SKWT7mdGnQKMrv6c26d=iWdNPG7f1VQ@...l.gmail.com>
Subject: Re: [RFC v3 5/8] dmabuf: Add gpu cgroup charge transfer function

On Tue, Mar 22, 2022 at 2:52 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Mon, Mar 21, 2022 at 04:54:26PM -0700, "T.J. Mercier"
> <tjmercier@...gle.com> wrote:
> > Since the charge is duplicated in two cgroups for a short period
> > before it is uncharged from the source cgroup I guess the situation
> > you're thinking about is a global (or common ancestor) limit?
>
> The common ancestor was on my mind (after the self-shortcut).
>
> > I can see how that would be a problem for transfers done this way and
> > an alternative would be to swap the order of the charge operations:
> > first uncharge, then try_charge. To be certain the uncharge is
> > reversible if the try_charge fails, I think I'd need either a mutex
> > used at all gpucg_*charge call sites or access to the gpucg_mutex,
>
> Yes, that'd provide safe conditions for such operations, although I'm
> not sure these special types of memory can afford global lock on their
> fast paths.

I have a benchmark I think is suitable, so let me try this change to
the transfer implementation and see how it compares.

>
> > which implies adding transfer support to gpu.c as part of the gpucg_*
> > API itself and calling it here. Am I following correctly here?
>
> My idea was to provide a special API (apart from
> gpucp_{try_charge,uncharge}) to facilitate transfers...
>
> > This series doesn't actually add limit support just accounting, but
> > I'd like to get it right here.
>
> ...which could be implemented (or changed) depending on how the charging
> is realized internally.
>
>
> Michal