linux-kernel - Re: [PATCH v9 04/13] task_isolation: add initial support

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Date:	Tue, 5 Jul 2016 12:47:08 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Chris Metcalf <cmetcalf@...lanox.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-doc@...r.kernel.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 04/13] task_isolation: add initial support

On Tue, 5 Jul 2016, Frederic Weisbecker wrote:

> > >>That's true, but I'd argue the behavior in that case should be that you can
> > >>raise that kind of exception validly (so you can debug), and then you should
> > >>quiesce on return to userspace so the application doesn't see additional
> > >>exceptions.
> > >
> > >I don't see how we can quiesce such things.
> >
> > I'm imagining task A is in dataplane mode, and task B wants to debug
> > it by writing a breakpoint into its text.  When task A hits the
> > breakpoint, it will enter the kernel, and hold there while task B
> > pokes at it with ptrace.  When task A finally is allowed to return to
> > userspace, it should quiesce before entering userspace in case any
> > timer interrupts got scheduled (again, maybe due to softirqs or
> > whatever, or random other kernel activity targeting that core while it
> > was in the kernel, or whatever).  This is just the same kind of
> > quiescing we do on return from the initial prctl().
>
> Well again I think it shouldn't happen. Quiescing should be done once
> and for all.

For debugging something like that would be helpful. And yes for the
realtime use cases quiescing is once and for all (until we end a different
operation mode if requested by the app)


> > >> - Soft mode (I don't think we want this) - like "no signal" except you don't even quiesce
> > >>   on return to userspace, and asynchronous interrupts don't even cause a signal.
> > >>   It's basically "best effort", just nohz_full plus the code that tries to get things
> > >>   like LRU or vmstat to run before returning to userspace.  I think there isn't enough
> > >>   "value add" to make this a separate mode, though.
> > >
> > >I can imagine HPC to be willing this mode.
> >
> > Yes, perhaps.  I'm not convinced we want to target HPC without a much
> > clearer sense of why this is better than nohz_full, though.  I fear
> > people might think "task isolation" is better by definition and not
> > think too much about it, but I'm really not sure it is better for the
> > HPC use case, necessarily.

HPC folks generally like to actually understand what is going on in order
to get the best performance. Just expose the knobs for us please.