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From: Peter Zijlstra <peterz@infradead.org>
To: "Michal Koutný" <mkoutny@suse.com>
Cc: Tejun Heo <tj@kernel.org>,
	joel@joelfernandes.org, chris.hyser@oracle.com,
	joshdon@google.com, mingo@kernel.org, vincent.guittot@linaro.org,
	valentin.schneider@arm.com, mgorman@suse.de,
	linux-kernel@vger.kernel.org, tglx@linutronix.de,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Zefan Li <lizefan.x@bytedance.com>
Subject: Re: [PATCH 0/9] sched: Core scheduling interfaces
Date: Thu, 8 Apr 2021 17:02:20 +0200	[thread overview]
Message-ID: <YG8a/LbO4PXFLEix@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YG8EYKgcwLCfIZAV@blackbook>

On Thu, Apr 08, 2021 at 03:25:52PM +0200, Michal Koutný wrote:
> On Wed, Apr 07, 2021 at 08:34:24PM +0200, Peter Zijlstra <peterz@infradead.org> wrote:
> > IMO as long as cgroups have that tasks file, you get to support people
> > using it. That means that tasks joining your cgroup need to 'inherit'
> > cgroup properties.
> The tasks file is consequence of binding this to cgroups, I'm one step
> back. Why to make "core isolation" a cgroup property?

Yeah, dunno, people asked for it. I'm just proposing an implementation
that, when given the need, seems to make sense and is internally
consistent.

> (I understand this could help "visualize" what the common domains are if
> cgroups were the only API but with prctl the structure can be
> arbitrarily modified anyway.)
> 
> 
> > Given something like:
> > 
> >         R
> >        / \
> >       A   B
> >          / \
> >         C   D
> Thanks for the example. 
> 
> > B group can set core_sched=1 and then all its (and its decendants) tasks
> > get to have the same (group) cookie and cannot share with others.
> The same could be achieved with the first task of group B allocating its
> new cookie which would be inherited in its descednants.

Except then the task can CLEAR its own cookie and escape the constraint.

> > In that scenario the D subtree is a restriction (doesn't share) with the
> > B subtree.
> This implies D's isolation from everything else too, not just B's
> members, no?

Correct. Look at it as a contraint on co-scheduling, you can never,
whatever you do, share an SMT sibling with someone outside your subtree.

> > And all of B is a restriction on all its tasks, including the Real-Time
> > task that set a task cookie, in that none of them can share with tasks
> > outside of B (including system tasks which are in R), irrespective of
> > what they do with their task cookie.
> IIUC, the equivalent restriction could be achieved with the PTRACE-like
> check in the prctl API too (with respectively divided uids).

I'm not sure I understand; if tasks in A and B are of the same user,
then ptrace will not help anything. And per the above, you always have
ptrace on yourself so you can escape your constraint per the above.

> I'm curious whether the cgroup API actually simplifies things that are
> possible with the clone/prctl API or allows anything that wouldn't be
> otherwise possible.

With the cgroup API it is impossible for a task to escape the cgroup
constraint. It can never share a core with anything not in the subtree.

This is not possible with just the task interface.

If this is actually needed I've no clue, IMO all of cgroups is not
needed :-) Clearly other people feel differently about that.


Much of this would go away if CLEAR were not possible I suppose. But
IIRC the idea was to let a task isolate itself temporarily, while doing
some sensitive thing (eg. encrypt an email) but otherwise not be
constrained. But I'm not sure I can remember all the various things
people wanted this crud for :/

  reply	other threads:[~2021-04-08 16:49 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-01 13:10 Peter Zijlstra
2021-04-01 13:10 ` [PATCH 1/9] sched: Allow sched_core_put() from atomic context Peter Zijlstra
2021-04-01 13:10 ` [PATCH 2/9] sched: Implement core-sched assertions Peter Zijlstra
2021-04-01 13:10 ` [PATCH 3/9] sched: Trivial core scheduling cookie management Peter Zijlstra
2021-04-01 20:04   ` Josh Don
2021-04-02  7:13     ` Peter Zijlstra
2021-04-01 13:10 ` [PATCH 4/9] sched: Default core-sched policy Peter Zijlstra
2021-04-21 13:33   ` Peter Zijlstra
2021-04-21 14:31     ` Chris Hyser
2021-04-01 13:10 ` [PATCH 5/9] sched: prctl() core-scheduling interface Peter Zijlstra
2021-04-07 17:00   ` Peter Zijlstra
2021-04-18  3:52     ` Joel Fernandes
2021-04-01 13:10 ` [PATCH 6/9] kselftest: Add test for core sched prctl interface Peter Zijlstra
2021-04-01 13:10 ` [PATCH 7/9] sched: Cgroup core-scheduling interface Peter Zijlstra
2021-04-02  0:34   ` Josh Don
2021-04-01 13:10 ` [PATCH 8/9] rbtree: Remove const from the rb_find_add() comparator Peter Zijlstra
2021-04-01 13:10 ` [PATCH 9/9] sched: prctl() and cgroup interaction Peter Zijlstra
2021-04-03  1:30   ` Josh Don
2021-04-06 15:12     ` Peter Zijlstra
2021-04-04 23:39 ` [PATCH 0/9] sched: Core scheduling interfaces Tejun Heo
2021-04-05 18:46   ` Joel Fernandes
2021-04-06 14:16     ` Tejun Heo
2021-04-18  1:35       ` Joel Fernandes
2021-04-19  9:00         ` Peter Zijlstra
2021-04-21 13:35           ` Peter Zijlstra
2021-04-21 14:45             ` Chris Hyser
2021-04-06 15:32   ` Peter Zijlstra
2021-04-06 16:08     ` Tejun Heo
2021-04-07 18:39       ` Peter Zijlstra
2021-04-07 16:50   ` Michal Koutný
2021-04-07 18:34     ` Peter Zijlstra
2021-04-08 13:25       ` Michal Koutný
2021-04-08 15:02         ` Peter Zijlstra [this message]
2021-04-09  0:16           ` Josh Don
2021-04-19 11:30       ` Tejun Heo
2021-04-20  1:17         ` Josh Don

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