linux-kernel - Re: [PATCH v8 04/14] task_isolation: add initial support

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Date:	Tue, 20 Oct 2015 13:56:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	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>,
	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>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 04/14] task_isolation: add initial support

On Tue, Oct 20, 2015 at 1:36 PM, Chris Metcalf <cmetcalf@...hip.com> wrote:
> +/*
> + * In task isolation mode we try to return to userspace only after
> + * attempting to make sure we won't be interrupted again.  To handle
> + * the periodic scheduler tick, we test to make sure that the tick is
> + * stopped, and if it isn't yet, we request a reschedule so that if
> + * another task needs to run to completion first, it can do so.
> + * Similarly, if any other subsystems require quiescing, we will need
> + * to do that before we return to userspace.
> + */
> +bool _task_isolation_ready(void)
> +{
> +       WARN_ON_ONCE(!irqs_disabled());
> +
> +       /* If we need to drain the LRU cache, we're not ready. */
> +       if (lru_add_drain_needed(smp_processor_id()))
> +               return false;
> +
> +       /* If vmstats need updating, we're not ready. */
> +       if (!vmstat_idle())
> +               return false;
> +
> +       /* If the tick is running, request rescheduling; we're not ready. */
> +       if (!tick_nohz_tick_stopped()) {
> +               set_tsk_need_resched(current);
> +               return false;
> +       }
> +
> +       return true;
> +}

I still don't get why this is a loop.

I would argue that this should simply drain the LRU, quiet vmstat, and
return.  If the tick isn't stopped, then there's a reason why it's not
stopped (which may involve having SCHED_OTHER tasks around, in which
case user code shouldn't do that or there should simply be a
requirement that isolation requires a real-time scheduler class).

BTW, should isolation just be a scheduler class (SCHED_ISOLATED)?

--Andy
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