linux-kernel - Re: Ping: [PATCH v15 00/13] support "task

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Date:   Tue, 13 Sep 2016 12:12:06 -0400
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     Francis Giraldeau <francis.giraldeau@...il.com>,
        Gilad Ben Yossef <giladb@...lanox.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>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Christoph Lameter <cl@...ux.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        <linux-doc@...r.kernel.org>, <linux-api@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: Ping: [PATCH v15 00/13] support "task_isolation" mode

Thanks for your explanation of the TIF_TASK_ISOLATION
flag being needed for x86  _TIF_WORK_SYSCALL_ENTRY.
It makes perfect sense in retrospect :-)

On 9/12/2016 8:20 PM, Francis Giraldeau wrote:
> On a side note, the NOSIG mode may be confusing for the users. At first,
> I was expecting that NOSIG behaves the same way as the normal task isolation
> mode. In the current situation, if the user wants the normal behavior, but
> does not care about the signal, the user must register an empty signal handler.

So, "normal behavior" isn't really well defined once we start
allowing empty signal handlers.  In particular, task isolation will
be turned off before invoking your signal handler, and if the
handler is empty, you just lose isolation and that's that.  By
contrast, the NOSIG mode will try to keep you isolated.

I'm definitely open to suggestions about how to frame the API
for NOSIG or equivalent modes.  What were you expecting to
be able to do by suppressing the signal, and how is NOSIG not
the thing you wanted?

> However, if I understand correctly, other settings beside NOHZ and isolcpus
> are required to support quiet CPUs, such as irq_affinity and rcu_nocb. It would
> be very convenient from the user point of view if these other settings were configure
> correctly.

I think it makes sense to move towards a mode where enabling
task_isolation sets up the rcu_nocbs and irq_affinity automatically,
rather than requiring users to understand all the fiddly configuration
and boot argument details.

> I can work on that and also write some doc (Documentation/task-isolation.txt ?).

Sure, documentation is always welcome!

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com