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Subject: Re: [RFC 00/60] Coscheduling for Linux
To:     =?UTF-8?Q?Jan_H=2e_Sch=c3=b6nherr?= <jschoenh@amazon.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org
References: <20180907214047.26914-1-jschoenh@amazon.de>
From:   Subhra Mazumdar <subhra.mazumdar@oracle.com>
Message-ID: <65abfba5-7c51-fd99-898e-f6e74969fea3@oracle.com>
Date:   Fri, 26 Oct 2018 16:05:10 -0700
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> D) What can I *not* do with this?
> ---------------------------------
>
> Besides the missing load-balancing within coscheduled task-groups, this
> implementation has the following properties, which might be considered
> short-comings.
>
> This particular implementation focuses on SCHED_OTHER tasks managed by CFS
> and allows coscheduling them. Interrupts as well as tasks in higher
> scheduling classes are currently out-of-scope: they are assumed to be
> negligible interruptions as far as coscheduling is concerned and they do
> *not* cause a preemption of a whole group. This implementation could be
> extended to cover higher scheduling classes. Interrupts, however, are an
> orthogonal issue.
>
> The collective context switch from one coscheduled set of tasks to another
> -- while fast -- is not atomic. If a use-case needs the absolute guarantee
> that all tasks of the previous set have stopped executing before any task
> of the next set starts executing, an additional hand-shake/barrier needs to
> be added.
>
The leader doesn't kick the other cpus _immediately_ to switch to a
different cosched group. So threads from previous cosched group will keep
running in other HTs till their sched_slice is over (in worst case). This
can still keep the window of L1TF vulnerability open?