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From: Frederic Weisbecker <fweisbec@gmail.com>
To: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Gilad Ben Yossef <giladb@ezchip.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Rik van Riel <riel@redhat.com>, Tejun Heo <tj@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Christoph Lameter <cl@linux.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Andy Lutomirski <luto@amacapital.net>,
	linux-doc@vger.kernel.org, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 04/14] task_isolation: add initial support
Date: Thu, 28 Jan 2016 17:38:24 +0100	[thread overview]
Message-ID: <20160128163822.GD25866@lerouge> (raw)
In-Reply-To: <562FA8FD.8080502@ezchip.com>

On Tue, Oct 27, 2015 at 12:40:29PM -0400, Chris Metcalf wrote:
> On 10/21/2015 12:12 PM, Frederic Weisbecker wrote:
> >On Tue, Oct 20, 2015 at 04:36:02PM -0400, Chris Metcalf wrote:
> >>+/*
> >>+ * This routine controls whether we can enable task-isolation mode.
> >>+ * The task must be affinitized to a single nohz_full 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 ||
> >I think you'll have to make sure the task can not be concurrently reaffined
> >to more CPUs. This may involve setting task_isolation_flags under the runqueue
> >lock and thus move that tiny part to the scheduler code. And then we must forbid
> >changing the affinity while the task has the isolation flag, or deactivate the flag.
> >
> >In any case this needs some synchronization.
> 
> Well, as the comment says, this is not intended as a hard guarantee.
> As written, it might race with a concurrent sched_setaffinity(), but
> then again, it also is totally OK as written for sched_setaffinity() to
> change it away after the prctl() is complete, so it's not necessary to
> do any explicit synchronization.
> 
> This harks back again to the whole "polite vs aggressive" issue with
> how we envision task isolation.
> 
> The "polite" model basically allows you to set up the conditions for
> task isolation to be useful, and then if they are useful, great! What
> you're suggesting here is a bit more of the "aggressive" model, where
> we actually fail sched_setaffinity() either for any cpumask after
> task isolation is set, or perhaps just for resetting it to housekeeping
> cores.  (Note that we could in principle use PF_NO_SETAFFINITY to
> just hard fail all attempts to call sched_setaffinity once we enable
> task isolation, so we don't have to add more mechanism on that path.)
> 
> I'm a little reluctant to ever fail sched_setaffinity() based on the
> task isolation status with the current "polite" model, since an
> unprivileged application can set up for task isolation, and then
> presumably no one can override it via sched_setaffinity() from another
> task.  (I suppose you could do some kind of permissions-based thing
> where root can always override it, or some suitable capability, etc.,
> but I feel like that gets complicated quickly, for little benefit.)
> 
> The alternative you mention is that if the task is re-affinitized, it
> loses its task-isolation status, and that also seems like an unfortunate
> API, since if you are setting it with prctl(), it's really cleanest just to
> only be able to unset it with prctl() as well.
> 
> I think given the current "polite" API, the only question is whether in
> fact *no* initial test is the best thing, or if an initial test (as
> introduced
> in the v8 version) is defensible just as a help for catching an obvious
> mistake in setting up your task isolation.  I decided the advantage
> of catching the mistake were more important than the "API purity"
> of being 100% consistent in how we handled the interactions between
> affinity and isolation, but I am certainly open to argument on that one.
> 
> Meanwhile I think it still feels like the v8 code is the best compromise.

So what is the way to deal with a migration for example? When the task wakes
up on the non-isolated CPU, it gets warned or killed?

> 
> >>+	/* If the tick is running, request rescheduling; we're not ready. */
> >>+	if (!tick_nohz_tick_stopped()) {
> >Note that this function tells whether the tick is in dynticks mode, which means
> >the tick currently only run on-demand. But it's not necessarily completely stopped.
> 
> I think in fact this is the semantics we want (and that people requested),
> e.g. if the user requests an alarm(), we may still be ticking even though
> tick_nohz_tick_stopped() is true, but that test is still the right condition
> to use to return to user space, since the user explicitly requested the
> alarm.

It seems to break the initial purpose. If your task really doesn't want to be
disturbed, it simply can't arm a timer. tick_nohz_tick_stopped() is really no
other indication than the CPU trying to do its best to delay the next tick. But
that next tick could be re-armed every two msecs for example. Worse yet, if the
tick has been stopped and finally issues a timer that rearms itself every 1 msec,
tick_nohz_tick_stopped() will still be true.

Thanks.

> 
> >I think we should rename that function and the field it refers to.
> 
> Sounds like a good idea.
> 
> -- 
> Chris Metcalf, EZChip Semiconductor
> http://www.ezchip.com
> 

  reply	other threads:[~2016-01-28 16:38 UTC|newest]

Thread overview: 236+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-05-08 17:58 [PATCH 0/6] support "dataplane" mode for nohz_full Chris Metcalf
2015-05-08 17:58 ` [PATCH 1/6] nohz_full: add support for "dataplane" mode Chris Metcalf
2015-05-08 17:58 ` [PATCH 2/6] nohz: dataplane: allow tick to be fully disabled for dataplane Chris Metcalf
2015-05-12  9:26   ` Peter Zijlstra
2015-05-12 13:12     ` Paul E. McKenney
2015-05-14 20:55       ` Chris Metcalf
2015-05-08 17:58 ` [PATCH 3/6] dataplane nohz: run softirqs synchronously on user entry Chris Metcalf
2015-05-09  7:04   ` Mike Galbraith
2015-05-11 20:13     ` Chris Metcalf
2015-05-12  2:21       ` Mike Galbraith
2015-05-12  9:28       ` Peter Zijlstra
2015-05-12  9:32       ` Peter Zijlstra
2015-05-12 13:08         ` Paul E. McKenney
2015-05-08 17:58 ` [PATCH 4/6] nohz: support PR_DATAPLANE_QUIESCE Chris Metcalf
2015-05-12  9:33   ` Peter Zijlstra
2015-05-12  9:50     ` Ingo Molnar
2015-05-12 10:38       ` Peter Zijlstra
2015-05-12 12:52         ` Ingo Molnar
2015-05-13  4:35           ` Andy Lutomirski
2015-05-13 17:51             ` Paul E. McKenney
2015-05-14 20:55               ` Chris Metcalf
2015-05-14 20:54     ` Chris Metcalf
2015-05-08 17:58 ` [PATCH 5/6] nohz: support PR_DATAPLANE_STRICT mode Chris Metcalf
2015-05-09  7:28   ` Andy Lutomirski
2015-05-09 10:37     ` Gilad Ben Yossef
2015-05-11 19:13     ` Chris Metcalf
2015-05-11 22:28       ` Andy Lutomirski
2015-05-12 21:06         ` Chris Metcalf
2015-05-12 22:23           ` Andy Lutomirski
2015-05-15 21:25             ` Chris Metcalf
2015-05-12  9:38   ` Peter Zijlstra
2015-05-12 13:20     ` Paul E. McKenney
2015-05-08 17:58 ` [PATCH 6/6] nohz: add dataplane_debug boot flag Chris Metcalf
2015-05-08 21:18 ` [PATCH 0/6] support "dataplane" mode for nohz_full Andrew Morton
2015-05-08 21:22   ` Steven Rostedt
2015-05-08 23:11     ` Chris Metcalf
2015-05-08 23:19       ` Andrew Morton
2015-05-09  7:05         ` Ingo Molnar
2015-05-09  7:19           ` Andy Lutomirski
2015-05-11 19:54             ` Chris Metcalf
2015-05-11 22:15               ` Andy Lutomirski
     [not found]             ` <55510885.9070101@ezchip.com>
2015-05-12 13:18               ` Paul E. McKenney
2015-05-09  7:19           ` Mike Galbraith
2015-05-09 10:18             ` Gilad Ben Yossef
2015-05-11 12:57           ` Steven Rostedt
2015-05-11 15:36             ` Frederic Weisbecker
2015-05-11 19:19               ` Mike Galbraith
2015-05-11 19:25                 ` Chris Metcalf
2015-05-12  1:47                   ` Mike Galbraith
2015-05-12  4:35                     ` Mike Galbraith
2015-05-15 15:05                     ` Chris Metcalf
2015-05-15 18:44                       ` Mike Galbraith
2015-05-26 19:51                         ` Chris Metcalf
2015-05-27  3:28                           ` Mike Galbraith
2015-05-11 17:19             ` Paul E. McKenney
2015-05-11 17:27               ` Andrew Morton
2015-05-11 17:33                 ` Frederic Weisbecker
2015-05-11 18:00                   ` Steven Rostedt
2015-05-11 18:09                     ` Chris Metcalf
2015-05-11 18:36                       ` Steven Rostedt
2015-05-12  9:10                       ` CONFIG_ISOLATION=y (was: [PATCH 0/6] support "dataplane" mode for nohz_full) Ingo Molnar
2015-05-12 11:48                         ` Peter Zijlstra
2015-05-12 12:34                           ` Ingo Molnar
2015-05-12 12:39                             ` Peter Zijlstra
2015-05-12 12:43                               ` Ingo Molnar
2015-05-12 15:36                             ` Frederic Weisbecker
2015-05-12 21:05                         ` CONFIG_ISOLATION=y Chris Metcalf
2015-05-12 10:46             ` [PATCH 0/6] support "dataplane" mode for nohz_full Peter Zijlstra
2015-05-15 15:10               ` Chris Metcalf
2015-05-15 21:26 ` [PATCH v2 0/5] support "cpu_isolated" " Chris Metcalf
2015-05-15 21:27   ` [PATCH v2 1/5] nohz_full: add support for "cpu_isolated" mode Chris Metcalf
2015-05-15 21:27     ` [PATCH v2 2/5] nohz: support PR_CPU_ISOLATED_STRICT mode Chris Metcalf
2015-05-15 21:27     ` [PATCH v2 3/5] nohz: cpu_isolated strict mode configurable signal Chris Metcalf
2015-05-15 21:27     ` [PATCH v2 4/5] nohz: add cpu_isolated_debug boot flag Chris Metcalf
2015-05-15 21:27     ` [PATCH v2 5/5] nohz: cpu_isolated: allow tick to be fully disabled Chris Metcalf
2015-05-15 22:17     ` [PATCH v2 1/5] nohz_full: add support for "cpu_isolated" mode Thomas Gleixner
2015-05-28 20:38       ` Chris Metcalf
2015-06-03 15:29   ` [PATCH v3 0/5] support "cpu_isolated" mode for nohz_full Chris Metcalf
2015-06-03 15:29     ` [PATCH v3 1/5] nohz_full: add support for "cpu_isolated" mode Chris Metcalf
2015-06-03 15:29     ` [PATCH v3 2/5] nohz: support PR_CPU_ISOLATED_STRICT mode Chris Metcalf
2015-06-03 15:29     ` [PATCH v3 3/5] nohz: cpu_isolated strict mode configurable signal Chris Metcalf
2015-06-03 15:29     ` [PATCH v3 4/5] nohz: add cpu_isolated_debug boot flag Chris Metcalf
2015-06-03 15:29     ` [PATCH v3 5/5] nohz: cpu_isolated: allow tick to be fully disabled Chris Metcalf
2015-07-13 19:57     ` [PATCH v4 0/5] support "cpu_isolated" mode for nohz_full Chris Metcalf
2015-07-13 19:57       ` [PATCH v4 1/5] nohz_full: add support for "cpu_isolated" mode Chris Metcalf
2015-07-13 20:40         ` Andy Lutomirski
2015-07-13 21:01           ` Chris Metcalf
2015-07-13 21:45             ` Andy Lutomirski
2015-07-21 19:10               ` Chris Metcalf
2015-07-21 19:26                 ` Andy Lutomirski
2015-07-21 20:36                   ` Paul E. McKenney
2015-07-22 13:57                     ` Christoph Lameter
2015-07-22 19:28                       ` Paul E. McKenney
2015-07-22 20:02                         ` Christoph Lameter
2015-07-24 20:21                           ` Chris Metcalf
2015-07-24 20:22                   ` Chris Metcalf
2015-07-24 14:03                 ` Frederic Weisbecker
2015-07-24 20:19                   ` Chris Metcalf
2015-07-24 13:27         ` Frederic Weisbecker
2015-07-24 20:21           ` Chris Metcalf
2015-07-13 19:57       ` [PATCH v4 2/5] nohz: support PR_CPU_ISOLATED_STRICT mode Chris Metcalf
2015-07-13 21:47         ` Andy Lutomirski
2015-07-21 19:34           ` Chris Metcalf
2015-07-21 19:42             ` Andy Lutomirski
2015-07-24 20:29               ` Chris Metcalf
2015-07-13 19:57       ` [PATCH v4 3/5] nohz: cpu_isolated strict mode configurable signal Chris Metcalf
2015-07-13 19:58       ` [PATCH v4 4/5] nohz: add cpu_isolated_debug boot flag Chris Metcalf
2015-07-13 19:58       ` [PATCH v4 5/5] nohz: cpu_isolated: allow tick to be fully disabled Chris Metcalf
2015-07-28 19:49       ` [PATCH v5 0/6] support "cpu_isolated" mode for nohz_full Chris Metcalf
2015-07-28 19:49         ` [PATCH v5 1/6] vmstat: provide a function to quiet down the diff processing Chris Metcalf
2015-07-28 19:49         ` [PATCH v5 2/6] cpu_isolated: add initial support Chris Metcalf
2015-08-12 16:00           ` Frederic Weisbecker
2015-08-12 18:22             ` Chris Metcalf
2015-08-26 15:26               ` Frederic Weisbecker
2015-08-26 15:55                 ` Chris Metcalf
2015-07-28 19:49         ` [PATCH v5 3/6] cpu_isolated: support PR_CPU_ISOLATED_STRICT mode Chris Metcalf
2015-07-28 19:49         ` [PATCH v5 4/6] cpu_isolated: provide strict mode configurable signal Chris Metcalf
2015-07-28 19:49         ` [PATCH v5 5/6] cpu_isolated: add debug boot flag Chris Metcalf
2015-07-28 19:49         ` [PATCH v5 6/6] nohz: cpu_isolated: allow tick to be fully disabled Chris Metcalf
2015-08-25 19:55         ` [PATCH v6 0/6] support "task_isolated" mode for nohz_full Chris Metcalf
2015-08-25 19:55           ` [PATCH v6 1/6] vmstat: provide a function to quiet down the diff processing Chris Metcalf
2015-08-25 19:55           ` [PATCH v6 2/6] task_isolation: add initial support Chris Metcalf
2015-08-25 19:55           ` [PATCH v6 3/6] task_isolation: support PR_TASK_ISOLATION_STRICT mode Chris Metcalf
2015-08-26 10:36             ` Will Deacon
2015-08-26 15:10               ` Chris Metcalf
2015-09-02 10:13                 ` Will Deacon
2015-08-28 15:31               ` [PATCH v6.1 " Chris Metcalf
2015-08-25 19:55           ` [PATCH v6 4/6] task_isolation: provide strict mode configurable signal Chris Metcalf
2015-08-28 19:22             ` Andy Lutomirski
2015-09-02 18:38               ` [PATCH v6.2 3/6] task_isolation: support PR_TASK_ISOLATION_STRICT mode Chris Metcalf
2015-08-25 19:55           ` [PATCH v6 5/6] task_isolation: add debug boot flag Chris Metcalf
2015-08-25 19:55           ` [PATCH v6 6/6] nohz: task_isolation: allow tick to be fully disabled Chris Metcalf
2015-09-28 15:17           ` [PATCH v7 00/11] support "task_isolated" mode for nohz_full Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 01/11] vmstat: provide a function to quiet down the diff processing Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 02/11] task_isolation: add initial support Chris Metcalf
2015-10-01 12:14               ` Frederic Weisbecker
2015-10-01 12:18                 ` Thomas Gleixner
2015-10-01 12:23                   ` Frederic Weisbecker
2015-10-01 12:31                     ` Thomas Gleixner
2015-10-01 17:02                   ` Chris Metcalf
2015-10-01 21:20                     ` Thomas Gleixner
2015-10-02 17:15                       ` Chris Metcalf
2015-10-02 19:02                         ` Thomas Gleixner
2015-10-01 19:25                 ` Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 03/11] task_isolation: support PR_TASK_ISOLATION_STRICT mode Chris Metcalf
2015-09-28 20:51               ` Andy Lutomirski
2015-09-28 21:54                 ` Chris Metcalf
2015-09-28 22:38                   ` Andy Lutomirski
2015-09-29 17:35                     ` Chris Metcalf
2015-09-29 17:46                       ` Andy Lutomirski
2015-09-29 17:57                         ` Chris Metcalf
2015-09-29 18:00                           ` Andy Lutomirski
2015-10-01 19:25                             ` Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 04/11] task_isolation: provide strict mode configurable signal Chris Metcalf
2015-09-28 20:54               ` Andy Lutomirski
2015-09-28 21:54                 ` Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 05/11] task_isolation: add debug boot flag Chris Metcalf
2015-09-28 20:59               ` Andy Lutomirski
2015-09-28 21:55                 ` Chris Metcalf
2015-09-28 22:40                   ` Andy Lutomirski
2015-09-29 17:35                     ` Chris Metcalf
2015-10-05 17:07               ` Luiz Capitulino
2015-10-08  0:33                 ` Chris Metcalf
2015-10-08 20:28                   ` Luiz Capitulino
2015-09-28 15:17             ` [PATCH v7 06/11] nohz: task_isolation: allow tick to be fully disabled Chris Metcalf
2015-09-28 20:40               ` Andy Lutomirski
2015-10-01 13:07                 ` Frederic Weisbecker
2015-10-01 14:13                   ` Thomas Gleixner
2015-09-28 15:17             ` [PATCH v7 07/11] arch/x86: enable task isolation functionality Chris Metcalf
2015-09-28 20:59               ` Andy Lutomirski
2015-09-28 21:57                 ` Chris Metcalf
2015-09-28 22:43                   ` Andy Lutomirski
2015-09-29 17:42                     ` Chris Metcalf
2015-09-29 17:57                       ` Andy Lutomirski
2015-09-30 20:25                         ` Thomas Gleixner
2015-09-30 20:58                           ` Chris Metcalf
2015-09-30 22:02                             ` Thomas Gleixner
2015-09-30 22:11                               ` Andy Lutomirski
2015-10-01  8:12                                 ` Thomas Gleixner
2015-10-01  9:08                                   ` Christoph Lameter
2015-10-01 10:10                                     ` Thomas Gleixner
2015-10-01 19:25                                   ` Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 08/11] arch/arm64: adopt prepare_exit_to_usermode() model from x86 Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 09/11] arch/arm64: enable task isolation functionality Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 10/11] arch/tile: adopt prepare_exit_to_usermode() model from x86 Chris Metcalf
2015-09-28 15:17             ` [PATCH v7 11/11] arch/tile: enable task isolation functionality Chris Metcalf
2015-10-20 20:35             ` [PATCH v8 00/14] support "task_isolation" mode for nohz_full Chris Metcalf
2015-10-20 20:35               ` [PATCH v8 01/14] vmstat: provide a function to quiet down the diff processing Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 02/14] vmstat: add vmstat_idle function Chris Metcalf
2015-10-20 20:45                 ` Christoph Lameter
2015-10-20 20:36               ` [PATCH v8 03/14] lru_add_drain_all: factor out lru_add_drain_needed Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 04/14] task_isolation: add initial support Chris Metcalf
2015-10-20 20:56                 ` Andy Lutomirski
2015-10-20 21:20                   ` Chris Metcalf
2015-10-20 21:26                     ` Andy Lutomirski
2015-10-21  0:29                       ` Steven Rostedt
2015-10-26 20:19                         ` Chris Metcalf
2015-10-26 21:13                         ` Chris Metcalf
2015-10-26 20:32                       ` Chris Metcalf
2015-10-21 16:12                 ` Frederic Weisbecker
2015-10-27 16:40                   ` Chris Metcalf
2016-01-28 16:38                     ` Frederic Weisbecker [this message]
2016-02-11 19:58                       ` Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 05/14] task_isolation: support PR_TASK_ISOLATION_STRICT mode Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 06/14] task_isolation: provide strict mode configurable signal Chris Metcalf
2015-10-21  0:56                 ` Steven Rostedt
2015-10-21  1:30                   ` Chris Metcalf
2015-10-21  1:41                     ` Steven Rostedt
2015-10-21  1:42                     ` Andy Lutomirski
2015-10-21  6:41                       ` Gilad Ben Yossef
2015-10-21 18:53                         ` Andy Lutomirski
2015-10-22 20:44                           ` Chris Metcalf
2015-10-22 21:00                             ` Andy Lutomirski
2015-10-27 19:37                               ` Chris Metcalf
2015-10-24  9:16                           ` Gilad Ben Yossef
2015-10-20 20:36               ` [PATCH v8 07/14] task_isolation: add debug boot flag Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 08/14] nohz_full: allow disabling the 1Hz minimum tick at boot Chris Metcalf
2015-10-20 21:03                 ` Frederic Weisbecker
2015-10-20 21:18                   ` Chris Metcalf
2015-10-21  0:59                     ` Steven Rostedt
2015-10-21  6:56                   ` Gilad Ben Yossef
2015-10-21 14:28                   ` Christoph Lameter
2015-10-21 15:35                     ` Frederic Weisbecker
2015-10-20 20:36               ` [PATCH v8 09/14] arch/x86: enable task isolation functionality Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 10/14] arch/arm64: adopt prepare_exit_to_usermode() model from x86 Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 11/14] arch/arm64: enable task isolation functionality Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 12/14] arch/tile: adopt prepare_exit_to_usermode() model from x86 Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 13/14] arch/tile: turn off timer tick for oneshot_stopped state Chris Metcalf
2015-10-20 20:36               ` [PATCH v8 14/14] arch/tile: enable task isolation functionality Chris Metcalf
2015-10-21 12:39               ` [PATCH v8 00/14] support "task_isolation" mode for nohz_full Peter Zijlstra
2015-10-22 20:31                 ` Chris Metcalf
2015-10-23  2:33                   ` Frederic Weisbecker
2015-10-23  8:49                     ` Peter Zijlstra
2015-10-23 13:29                       ` Frederic Weisbecker
2015-10-23  9:04                   ` Peter Zijlstra
2015-10-23 11:52                     ` Theodore Ts'o

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