linux-kernel - Re: [PATCH V2 3/3] sched/deadline: Tracepoints for deadline scheduler

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Date:	Tue, 29 Mar 2016 15:25:10 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Juri Lelli <juri.lelli@....com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH V2 3/3] sched/deadline: Tracepoints for deadline
 scheduler

On Tue, 29 Mar 2016 16:12:38 -0300
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> On 03/29/2016 02:13 PM, Steven Rostedt wrote:
> >>           <idle>-0     [007] d..3 78377.688969: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=b next_pid=18973 next_prio=-1  
> >> >                b-18973 [007] d..3 78377.688979: sched_deadline_block: now=78377.688976271 deadline=78377.718945137 remaining_runtime=9968866
> >> >                b-18973 [007] d..3 78377.688981: sched_switch: prev_comm=b prev_pid=18973 prev_prio=-1 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120  
> > Why did it go to sleep? The above is still not very useful. What do you
> > mean "blocking on a system call"?  
> 
> A task can go can to sleep in a blocking system call, like waiting
> a network packet, or any other external event.

Note, waiting for a network packet or some other external event is a
userspace call. A schedule out in 'S' state means exactly that. But
I hate the term "blocked" because that is more like waiting for
something else to finish (like blocked on a lock). In which case, if
that did happen, the state would be "D" not "S".

"S" is basically "sleeping" and it gets woken up by some other event. A
slight difference to the term "blocked".

> 
> The "block state" is a possible state of a task running in the deadline
> scheduler. It means that a task voluntarily left the processor, not
> by calling sched_yield(), but by blocking (or sleeping) waiting another
> event.
> 
> This state is described in the Figure 2 of the article "Deadline
> scheduling in the Linux kernel", available at:
> http://onlinelibrary.wiley.com/doi/10.1002/spe.2335/abstract

Bah, confusing terminology.

> 
> The block state affects the replenishment of the task, and that is why
> it is different of both yeild and throttle. If the task blocks and is
> awakened prior to the deadline, the task's runtime will not be
> replenished. On the other hand it will. For example:
> 
> Blocking, and then waking up after the deadline:
>                b-5152  [007] d..3  3983.376428: sched_deadline_replenish: comm=b pid=5152 now=3983.376425148 deadline=3983.406425148 runtime=10000000
>                b-5152  [007] d..3  3983.376515: sched_deadline_block: now=3983.376511101 deadline=3983.406425148 remaining_runtime=9909566
>                b-5152  [007] d..3  3983.376529: sched_switch: prev_comm=b prev_pid=5152 prev_prio=-1 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120
> 
>           <idle>-0     [007] d.h4  3983.476592: sched_deadline_replenish: comm=b pid=5152 now=3983.476589573 deadline=3983.506589573 runtime=10000000
>           <idle>-0     [007] dNh4  3983.476596: sched_wakeup: comm=b pid=5152 prio=-1 target_cpu=007
>           <idle>-0     [007] d..3  3983.476648: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=b next_pid=5152 next_prio=-1
>                b-5152  [007] d..3  3983.476660: sched_deadline_block: now=3983.476656613 deadline=3983.506589573 remaining_runtime=9932960
>                b-5152  [007] d..3  3983.476663: sched_switch: prev_comm=b prev_pid=5152 prev_prio=-1 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120
> 
> 
> Blocking, and then waking up before the deadline:
>                b-5139  [007] d..3  3964.148290: sched_deadline_replenish: comm=b pid=5139 now=3964.148285227 deadline=3964.178285227 runtime=10000000
>                b-5139  [007] d..3  3964.148396: sched_deadline_block: now=3964.148385308 deadline=3964.178285227 remaining_runtime=9895243
>                b-5139  [007] d..3  3964.148400: sched_switch: prev_comm=b prev_pid=5139 prev_prio=-1 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120
> 
>           <idle>-0     [007] dNh5  3964.148411: sched_wakeup: comm=b pid=5139 prio=-1 target_cpu=007
>           <idle>-0     [007] d..3  3964.148419: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=b next_pid=5139 next_prio=-1
>                b-5139  [007] d..3  3964.148427: sched_deadline_block: now=3964.148426022 deadline=3964.178285227 remaining_runtime=9878164
>                b-5139  [007] d..3  3964.148429: sched_switch: prev_comm=b prev_pid=5139 prev_prio=-1 prev_state=S ==> next_comm=swapper/7 next_pid=0 next_prio=120
> 

I still fail to see the usefulness of the block tracepoint. I could
imagine that if we add the dynamic part of the sched_switch tracepoint
to include deadline and runtime, we would get the same information.

-- Steve