linux-kernel - Re: [PATCH v14 04/14] task_isolation: add initial support

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Message-ID: <20160811181132.GD4214@lerouge>
Date:	Thu, 11 Aug 2016 20:11:33 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Metcalf <cmetcalf@...lanox.com>
Cc:	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>,
	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>,
	Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v14 04/14] task_isolation: add initial support

On Tue, Aug 09, 2016 at 04:29:46PM -0400, Chris Metcalf wrote:
> +/*
> + * Each time we try to prepare for return to userspace in a process
> + * with task isolation enabled, we run this code to quiesce whatever
> + * subsystems we can readily quiesce to avoid later interrupts.
> + */
> +void task_isolation_enter(void)
> +{
> +	WARN_ON_ONCE(irqs_disabled());
> +
> +	/* Drain the pagevecs to avoid unnecessary IPI flushes later. */
> +	lru_add_drain();
> +
> +	/* Quieten the vmstat worker so it won't interrupt us. */
> +	quiet_vmstat_sync();

So, this is going to be called everytime we resume to userspace
while in task isolation mode, right?

Do we need to quiesce vmstat everytime before entering userspace?
I thought that vmstat only need to be offlined once and for all?

And how about lru?

> +
> +	/*
> +	 * Request rescheduling unless we are in full dynticks mode.
> +	 * We would eventually get pre-empted without this, and if
> +	 * there's another task waiting, it would run; but by
> +	 * explicitly requesting the reschedule, we may reduce the
> +	 * latency.  We could directly call schedule() here as well,
> +	 * but since our caller is the standard place where schedule()
> +	 * is called, we defer to the caller.
> +	 *
> +	 * A more substantive approach here would be to use a struct
> +	 * completion here explicitly, and complete it when we shut
> +	 * down dynticks, but since we presumably have nothing better
> +	 * to do on this core anyway, just spinning seems plausible.
> +	 */
> +	if (!tick_nohz_tick_stopped())
> +		set_tsk_need_resched(current);

Again, that won't help :-)