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Date:   Thu, 5 Nov 2020 10:52:05 -0500
From:   Joel Fernandes <joel@joelfernandes.org>
To:     "Li, Aubrey" <aubrey.li@linux.intel.com>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, tglx@linutronix.de,
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        torvalds@linux-foundation.org, fweisbec@gmail.com,
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        Valentin Schneider <valentin.schneider@arm.com>,
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        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
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        Chen Yu <yu.c.chen@intel.com>,
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        Alexandre Chartre <alexandre.chartre@oracle.com>,
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        Dhaval Giani <dhaval.giani@oracle.com>,
        Junaid Shahid <junaids@google.com>, jsbarnes@google.com,
        chris.hyser@oracle.com, "Paul E. McKenney" <paulmck@kernel.org>,
        Tim Chen <tim.c.chen@intel.com>
Subject: Re: [PATCH v8 -tip 19/26] sched: Add a second-level tag for nested
 CGroup usecase
Message-ID: <20201105155205.GC2656962@google.com>
References: <20201020014336.2076526-1-joel@joelfernandes.org>
 <20201020014336.2076526-20-joel@joelfernandes.org>
 <6c07e70d-52f2-69ff-e1fa-690cd2c97f3d@linux.intel.com>
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On Wed, Oct 28, 2020 at 02:23:02PM +0800, Li, Aubrey wrote:
> On 2020/10/20 9:43, Joel Fernandes (Google) wrote:
> > Google has a usecase where the first level tag to tag a CGroup is not
> > sufficient. So, a patch is carried for years where a second tag is added which
> > is writeable by unprivileged users.
> > 
> > Google uses DAC controls to make the 'tag' possible to set only by root while
> > the second-level 'color' can be changed by anyone. The actual names that
> > Google uses is different, but the concept is the same.
> > 
> > The hierarchy looks like:
> > 
> > Root group
> >    / \
> >   A   B    (These are created by the root daemon - borglet).
> >  / \   \
> > C   D   E  (These are created by AppEngine within the container).
> > 
> > The reason why Google has two parts is that AppEngine wants to allow a subset of
> > subcgroups within a parent tagged cgroup sharing execution. Think of these
> > subcgroups belong to the same customer or project. Because these subcgroups are
> > created by AppEngine, they are not tracked by borglet (the root daemon),
> > therefore borglet won't have a chance to set a color for them. That's where
> > 'color' file comes from. Color could be set by AppEngine, and once set, the
> > normal tasks within the subcgroup would not be able to overwrite it. This is
> > enforced by promoting the permission of the color file in cgroupfs.
> > 
> > The 'color' is a 8-bit value allowing for upto 256 unique colors. IMHO, having
> > more than these many CGroups sounds like a scalability issue so this suffices.
> > We steal the lower 8-bits of the cookie to set the color.
> > 
> 
> So when color = 0, tasks in group A C D can run together on the HTs in same core,
> And if I set the color of taskC in group C = 1, then taskC has a different cookie
> from taskA and taskD, so in terms of taskA, what's the difference between taskC
> and [taskB or taskE]? The color breaks the relationship that C belongs to A.

C does belong to A in the sense, A cannot share with B, this implies C can
never share with B. Setting C's color does not change that fact. So coloring
is irrelevant in your question.

Sure, A cannot share with C either after coloring, but that's irrelevant and
not the point of doing the coloring.

thanks,

 - Joel