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Date:   Fri, 5 Feb 2021 11:43:03 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Chris Hyser <chris.hyser@oracle.com>
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Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
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On Thu, Feb 04, 2021 at 03:52:55PM -0500, Chris Hyser wrote:

> A second complication was a decision that new processes (not threads) do not
> inherit their parents cookie. Thus forking is also not a means to share a
> cookie. Basically with a "from-only" interface, the new task would need to
> be modified to call prctl() itself. From-only also does not allow for
> setting a cookie on an unmodified already running task. This can be fixed by
> providing both a "to" and "from" sharing interface that allows helper
> programs to construct arbitrary configurations from unmodified programs.

Do we really want to inhibit on fork() or would exec() be a better
place? What about those applications that use fork() based workers?

> > Also, how do I set a unique cookie on myself with this interface?
> 
> The v10 patch still uses the overloaded v9 mechanism (which as mentioned
> above is if two tasks w/o cookies share a cookie they get a new shared
> unique cookie). Yes, that is clearly an inconsistency and kludgy. The
> mechanism is documented in the docs, but clearly not obvious from the

I've not seen a document so far (also, I'm not one to actually read
documentation, much preferring comments and Changelogs).

> So based on the above, how about we add a "create" to pair with "clear" and
> call it "create" vs "set" since we are creating a unique opaque cookie
> versus setting a particular value. And as mentioned, because one can't
> specify a cookie directly but only thru sharing relationships, we need both
> "to" and "from" to make it completely usable.
> 
> So we end up with something like this:
>     PR_SCHED_CORE_CREATE                       -- give yourself a unique cookie
>     PR_SCHED_CORE_CLEAR                        -- clear your core sched cookie
>     PR_SCHED_CORE_SHARE_FROM <src_task>        -- get their cookie for you
>     PR_SCHED_CORE_SHARE_TO   <dest_task>       -- push your cookie to them

I'm still wondering why we need _FROM/_TO. What exactly will we miss
with just _SHARE <pid>?

	current		arg_task
	<none>		<none>		-EDAFT
	<none>		<cookie>	current gets cookie
	<cookie>	<none>		arg_task gets cookie
	<cookie>	<cookie>	-EDAFTER

(I have a suspicion, but I want to see it spelled out).

Also, do we wants this interface to be able to work on processes? Things
like fcntl(F_SETOWN_EX) allow you to specify a PID type.

> An additional question is should the inheritability of a process' cookie be
> configurable? The current code gives the child process their own unique
> cookie if the parent had a cookie. That is useful in some cases, but many
> other configurations could be made much easier with inheritance.

What was the argument for not following the traditional fork() semantics
and inheriting everything?