Re: [PATCH 14/31] sched_ext: Implement BPF extensible scheduler class

[Rik van Riel]

On Tue, 2022-12-13 at 08:12 -1000, Tejun Heo wrote:
> Hello,
> 
> On Tue, Dec 13, 2022 at 11:55:10AM +0100, Peter Zijlstra wrote:
> > On Mon, Dec 12, 2022 at 11:33:12AM -1000, Tejun Heo wrote:
> > 
> > > Here, the way it's handled is a bit different, SCX has
> > > a watchdog mechanism implemented in "[PATCH 18/31] sched_ext:
> > > Implement
> > > runnable task stall watchdog", so if SCX tasks hang for whatever
> > > reason
> > > including being starved by CFS, it will get aborted and all tasks
> > > will be
> > > handed back to CFS. IOW, it's treated like any other BPF
> > > scheduler errors
> > > that can lead to stalls and recovered the same way.
> > 
> > That all sounds quite terrible.. :/
> 
> The main source of difference is that we can't implicitly trust the
> BPF
> scheduler and if it malfunctions or on user request, the system
> should
> always be recoverable, so there are some extra things which are
> inherently
> necessary to support that.
> 
That makes me wonder whether loading an SCX policy
should just have that policy take over all of the
SCHED_OTHER tasks by default, and have a failure of
the policy just return those tasks to CFS?

Having the two be operative at the same time seems
to be a cause of hard to resolve issues, while simply
running all non-RT tasks under the loadable policy
could simplify both internal kernel interfaces, as
well as externally visible effects?

-- 
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