Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 26 Mar 2018 11:54:51 -0400
From:   joe.korty@concurrent-rt.com
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Julia Cartwright <julia@ni.com>, <bigeasy@linutronix.de>,
        <mingo@redhat.com>, <tglx@linutronix.de>,
        <linux-rt-users@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RT] Defer migrate_enable migration while task state !=
 TASK_RUNNING
Message-ID: <20180326155451.GA16545@zipoli.concurrent-rt.com>
Reply-To: "Joe Korty" <joe.korty@concurrent-rt.com>
References: <20180323150959.GA16131@zipoli.concurrent-rt.com>
 <20180323165921.GG10942@jcartwri.amer.corp.natinst.com>
 <20180323172131.GA2670@zipoli.concurrent-rt.com>
 <20180326113515.720e7fb3@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180326113515.720e7fb3@gandalf.local.home>
User-Agent: Mutt/1.8.3 (2017-05-23)
Received-SPF: None (protection.outlook.com: concurrent-rt.com does not
 designate permitted sender hosts)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 26 Mar 2018 15:54:57.9098 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 64424a96-615e-4e38-5f0a-08d59331ed65
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 38747689-e6b0-4933-86c0-1116ee3ef93e
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR11MB2570
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Oh well.  Makes me wonder why might_sleep is testing for
!TASK_RUNNABLE though.

Thanks for the correction,
Joe


On Mon, Mar 26, 2018 at 11:35:15AM -0400, Steven Rostedt wrote:
> On Fri, 23 Mar 2018 13:21:31 -0400
> joe.korty@concurrent-rt.com wrote:
> 
> > My understanding is, in standard Linux and in rt, setting
> > task state to anything other than TASK_RUNNING in of itself
> > blocks preemption.
> 
> That is clearly false. The only thing that blocks preemption with a
> CONFIG_PREEMPT kernel is preempt_disable() and local_irq*() disabling.
> 
> (Note spin_locks call preempt_disable in non RT).
> 
> Otherwise, nothing will stop preemption.
> 
> >  A preemption is not really needed here
> > as it is expected that there is a schedule() written in that
> > will shortly be executed.  And if a 'involuntary schedule'
> > (ie, preemption) were allowed to occur between the task
> > state set and the schedule(), that would change the task
> > state back to TASK_RUNNING, which would cause the schedule
> > to NOP.  Thus we risk not having paused long enough here
> > for the condition we were waiting for to become true.
> 
> That is also incorrect. As Julia mentioned, a preemption keeps the
> state of the task.