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

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Message-ID: <20160119154214.GA2722@lerouge>
Date:	Tue, 19 Jan 2016 16:42:16 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
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>,
	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>,
	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 Mon, Jan 04, 2016 at 02:34:42PM -0500, Chris Metcalf wrote:
> diff --git a/kernel/isolation.c b/kernel/isolation.c
> new file mode 100644
> index 000000000000..68a9f7457bc0
> --- /dev/null
> +++ b/kernel/isolation.c
> @@ -0,0 +1,105 @@
> +/*
> + *  linux/kernel/isolation.c
> + *
> + *  Implementation for task isolation.
> + *
> + *  Distributed under GPLv2.
> + */
> +
> +#include <linux/mm.h>
> +#include <linux/swap.h>
> +#include <linux/vmstat.h>
> +#include <linux/isolation.h>
> +#include <linux/syscalls.h>
> +#include "time/tick-sched.h"
> +
> +cpumask_var_t task_isolation_map;
> +
> +/*
> + * Isolation requires both nohz and isolcpus support from the scheduler.
> + * We provide a boot flag that enables both for now, and which we can
> + * add other functionality to over time if needed.  Note that just
> + * specifying "nohz_full=... isolcpus=..." does not enable task isolation.
> + */
> +static int __init task_isolation_setup(char *str)
> +{
> +	alloc_bootmem_cpumask_var(&task_isolation_map);
> +	if (cpulist_parse(str, task_isolation_map) < 0) {
> +		pr_warn("task_isolation: Incorrect cpumask '%s'\n", str);
> +		return 1;
> +	}
> +
> +	alloc_bootmem_cpumask_var(&cpu_isolated_map);
> +	cpumask_copy(cpu_isolated_map, task_isolation_map);
> +
> +	alloc_bootmem_cpumask_var(&tick_nohz_full_mask);
> +	cpumask_copy(tick_nohz_full_mask, task_isolation_map);
> +	tick_nohz_full_running = true;

How about calling tick_nohz_full_setup() instead? I'd rather prefer
that nohz full implementation details stay in tick-sched.c

Also what happens if nohz_full= is given as well as task_isolation= ?
Don't we risk a memory leak and maybe breaking the fact that
(nohz_full & task_isolation != task_isolation) which is really a requirement?

> +
> +	return 1;
> +}
> +__setup("task_isolation=", task_isolation_setup);
> +
> +/*
> + * This routine controls whether we can enable task-isolation mode.
> + * The task must be affinitized to a single task_isolation core or we will
> + * return EINVAL.  Although the application could later re-affinitize
> + * to a housekeeping core and lose task isolation semantics, this
> + * initial test should catch 99% of bugs with task placement prior to
> + * enabling task isolation.
> + */
> +int task_isolation_set(unsigned int flags)
> +{
> +	if (cpumask_weight(tsk_cpus_allowed(current)) != 1 ||
> +	    !task_isolation_possible(smp_processor_id()))
> +		return -EINVAL;
> +
> +	current->task_isolation_flags = flags;
> +	return 0;
> +}

What if we concurrently change the task's affinity? Also it seems that preemption
isn't disabled, so we can also migrate concurrently. I'm surprised you haven't
seen warnings with smp_processor_id().

Also we should protect against task's affinity change when task_isolation_flags
is set.

> +
> +/*
> + * 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;
> +
> +	/* Request rescheduling unless we are in full dynticks mode. */
> +	if (!tick_nohz_tick_stopped()) {
> +		set_tsk_need_resched(current);

I'm not sure doing this will help getting the tick to get stopped.

> +		return false;
> +	}
> +
> +	return true;
> +}

Thanks!