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Date: Mon, 31 Mar 2014 08:38:13 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: fweisbec@...il.com, peterz@...radead.org,
linux-kernel@...r.kernel.org
Subject: Re: Promela/spin model for NO_HZ_FULL_SYSIDLE code
On Mon, Mar 31, 2014 at 02:02:23PM +0000, Mathieu Desnoyers wrote:
> ----- Original Message -----
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> > Cc: fweisbec@...il.com, peterz@...radead.org, linux-kernel@...r.kernel.org
> > Sent: Sunday, March 30, 2014 11:54:58 PM
> > Subject: Re: Promela/spin model for NO_HZ_FULL_SYSIDLE code
> >
> > On Mon, Mar 31, 2014 at 03:29:25AM +0000, Mathieu Desnoyers wrote:
> > > ----- Original Message -----
> > > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > > To: fweisbec@...il.com, "mathieu desnoyers"
> > > > <mathieu.desnoyers@...icios.com>, peterz@...radead.org
> > > > Cc: linux-kernel@...r.kernel.org
> > > > Sent: Sunday, March 30, 2014 7:08:56 PM
> > > > Subject: Promela/spin model for NO_HZ_FULL_SYSIDLE code
> > > >
> > > > For whatever it is worth, the following model claims safety and progress
> > > > for the sysidle state machine.
> > > >
> > > > Thoughts?
> > >
> > > Hi Paul,
> > >
> > > Please keep in mind that it's been a while since I've looked at Promela
> > > proofs, but a couple of questions come to mind. First, how is the execution
> > > script below checking for progress in any way ? I remember that we used
> > > the negation of a "_np" LTL claim to check for progress in the past.
> > > Moreover, I'd be much more comfortable seeing ifdefs in the code that
> > > inject
> > > known failures, and let them be triggered by error-triggering runs in the
> > > scripts (with e.g. -DINJECT_ERROR_XYZ), to confirm that this model actually
> > > catches known issues.
> >
> > Well, if I comment out the four "progress_" labels, it complains about
> > a non-progress cycle. So at least spin does pay attention to these. ;-)
> >
> > If I put the "progress_" labels back in, but change the check so as to
> > prevent the state machine from ever entering RCU_SYSIDLE_FULL_NOTED,
> > it cranks through 14M states before complaining about a non-progress cycle,
> > as expected.
> >
> > If I revert back to pristine state, and then comment out the statements
> > that reverts state back to RCU_SYSIDLE_NOT when exiting idle, an assert()
> > triggers, as expected.
> >
> > > If we can show that the model can detect a few failure modes, both for
> > > safety
> > > and progress, then my confidence level in the model will start to improve.
> > > ;-)
> >
> > Well, there probably is a bug in there somewhere, Murphy being who he is.
> > ;-)
>
> One failure mode your model seems to miss is when I comment the wakeup:
>
> /* If needed, wake up the timekeeper. */
> if
> #ifndef INJECT_NO_WAKEUP
> :: oldstate == RCU_SYSIDLE_FULL_NOTED ->
> wakeup_timekeeper = 1;
> #endif /* #ifndef INJECT_NO_WAKEUP */
> :: else -> skip;
> fi;
>
> Somehow, I feel I am doing something very nasty to the algorithm,
> but the model checker fails to find any kind of progress or safety
> issue. Any idea why ?
Hmmm... One reason is because I am not modeling the timekeeper thread
as waiting for the wakeup. Let me see what I can do about that...
Thanx, Paul
> Thanks,
>
> Mathieu
>
>
> >
> > Thanx, Paul
> >
> > > Thanks,
> > >
> > > Mathieu
> > >
> > > >
> > > > Thanx, Paul
> > > >
> > > > ------------------------------------------------------------------------
> > > > sysidle.sh
> > > > ------------------------------------------------------------------------
> > > > spin -a sysidle.spin
> > > > cc -DNP -o pan pan.c
> > > > # Fair scheduling to focus progress checks in timekeeper.
> > > > ./pan -f -l -m1280000 -w22
> > > >
> > > > ------------------------------------------------------------------------
> > > > sysidle.spin
> > > > ------------------------------------------------------------------------
> > > > /*
> > > > * Promela model for CONFIG_NO_HZ_FULL_SYSIDLE=y in the Linux kernel.
> > > > * This model assumes that the dyntick-idle bit manipulation works based
> > > > * on long usage, and substitutes a per-thread boolean "am_busy[]" array
> > > > * for the Linux kernel's dyntick-idle masks. The focus of this model
> > > > * is therefore on the state machine itself. Checks for both safety and
> > > > * forward progress.
> > > > *
> > > > * This program is free software; you can redistribute it and/or modify
> > > > * it under the terms of the GNU General Public License as published by
> > > > * the Free Software Foundation; either version 2 of the License, or
> > > > * (at your option) any later version.
> > > > *
> > > > * This program is distributed in the hope that it will be useful,
> > > > * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > > > * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > > > * GNU General Public License for more details.
> > > > *
> > > > * You should have received a copy of the GNU General Public License
> > > > * along with this program; if not, you can access it online at
> > > > * http://www.gnu.org/licenses/gpl-2.0.html.
> > > > *
> > > > * Copyright IBM Corporation, 2014
> > > > *
> > > > * Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > > */
> > > >
> > > > #define NUM_WORKERS 3
> > > >
> > > > byte wakeup_timekeeper = 0; /* Models rcu_kick_nohz_cpu(). */
> > > >
> > > > #define RCU_SYSIDLE_NOT 0 /* Some CPU is not idle. */
> > > > #define RCU_SYSIDLE_SHORT 1 /* All CPUs idle for brief period. */
> > > > #define RCU_SYSIDLE_LONG 2 /* All CPUs idle for long enough. */
> > > > #define RCU_SYSIDLE_FULL 3 /* All CPUs idle, ready for sysidle. */
> > > > #define RCU_SYSIDLE_FULL_NOTED 4 /* Actually entered sysidle state. */
> > > >
> > > > byte full_sysidle_state = RCU_SYSIDLE_NOT;
> > > >
> > > > byte am_busy[NUM_WORKERS]; /* Busy is similar to "not dyntick-idle". */
> > > > byte am_setup[NUM_WORKERS]; /* Setup means timekeeper knows I am not
> > > > idle. */
> > > >
> > > > /*
> > > > * Non-timekeeping CPU going into and out of dyntick-idle state.
> > > > */
> > > > proctype worker(byte me)
> > > > {
> > > > byte oldstate;
> > > >
> > > > do
> > > > :: 1 ->
> > > > /* Go idle. */
> > > > am_setup[me] = 0;
> > > > am_busy[me] = 0;
> > > >
> > > > /* Dyntick-idle in the following loop. */
> > > > do
> > > > :: 1 -> skip;
> > > > :: 1 -> break;
> > > > od;
> > > >
> > > > /* Exit idle loop, model getting out of dyntick idle state. */
> > > > am_busy[me] = 1;
> > > >
> > > > /* Get state out of full-system idle states. */
> > > > atomic {
> > > > oldstate = full_sysidle_state;
> > > > if
> > > > :: oldstate > RCU_SYSIDLE_SHORT ->
> > > > full_sysidle_state = RCU_SYSIDLE_NOT;
> > > > :: else -> skip;
> > > > fi;
> > > > }
> > > >
> > > > /* If needed, wake up the timekeeper. */
> > > > if
> > > > :: oldstate == RCU_SYSIDLE_FULL_NOTED ->
> > > > wakeup_timekeeper = 1;
> > > > :: else -> skip;
> > > > fi;
> > > >
> > > > /* Mark ourselves fully awake and operational. */
> > > > am_setup[me] = 1;
> > > >
> > > > /* We are fully awake, so timekeeper must not be asleep. */
> > > > assert(full_sysidle_state < RCU_SYSIDLE_FULL);
> > > >
> > > > /* Running in kernel in the following loop. */
> > > > do
> > > > :: 1 -> skip;
> > > > :: 1 -> break;
> > > > od;
> > > > od
> > > > }
> > > >
> > > > /*
> > > > * Are all the workers in dyntick-idle state?
> > > > */
> > > > #define check_idle() \
> > > > i = 0; \
> > > > idle = 1; \
> > > > do \
> > > > :: i < NUM_WORKERS -> \
> > > > if \
> > > > :: am_busy[i] == 1 -> idle = 0; \
> > > > :: else -> skip; \
> > > > fi; \
> > > > i++; \
> > > > :: i >= NUM_WORKERS -> break; \
> > > > od
> > > >
> > > > /*
> > > > * Timekeeping CPU.
> > > > */
> > > > proctype timekeeper()
> > > > {
> > > > byte i;
> > > > byte idle;
> > > > byte curstate;
> > > > byte newstate;
> > > >
> > > > do
> > > > :: 1 ->
> > > > /* Capture current state. */
> > > > check_idle();
> > > > curstate = full_sysidle_state;
> > > > newstate = curstate;
> > > >
> > > > /* Check for acceptance state. */
> > > > if
> > > > :: idle == 0 ->
> > > > progress_idle:
> > > > skip;
> > > > :: curstate == RCU_SYSIDLE_NOT ->
> > > > progress_idle_reset:
> > > > skip;
> > > > :: else -> skip;
> > > > fi;
> > > >
> > > > /* Manage state... */
> > > > if
> > > > :: idle == 1 && curstate < RCU_SYSIDLE_FULL_NOTED ->
> > > > /* Idle, advance to next state. */
> > > > atomic {
> > > > if
> > > > :: full_sysidle_state == curstate ->
> > > > newstate = curstate + 1;
> > > > full_sysidle_state = newstate;
> > > > :: else -> skip;
> > > > fi;
> > > > }
> > > > :: idle == 0 && full_sysidle_state >= RCU_SYSIDLE_LONG ->
> > > > /* Non-idle and state advanced, revert to base state. */
> > > > full_sysidle_state = RCU_SYSIDLE_NOT;
> > > > :: else -> skip;
> > > > fi;
> > > >
> > > > /* If in RCU_SYSIDLE_FULL_NOTED, wait to be awakened. */
> > > > do
> > > > :: newstate != RCU_SYSIDLE_FULL_NOTED &&
> > > > wakeup_timekeeper == 1 ->
> > > > assert(0); /* Should never get here. */
> > > > :: newstate != RCU_SYSIDLE_FULL_NOTED &&
> > > > wakeup_timekeeper == 0 ->
> > > > break;
> > > > :: newstate == RCU_SYSIDLE_FULL_NOTED &&
> > > > wakeup_timekeeper == 1 ->
> > > > progress_full_system_idle_1:
> > > > assert(full_sysidle_state == RCU_SYSIDLE_NOT);
> > > > wakeup_timekeeper = 0;
> > > > break;
> > > > :: newstate == RCU_SYSIDLE_FULL_NOTED &&
> > > > wakeup_timekeeper == 0 ->
> > > > progress_full_system_idle_2:
> > > >
> > > > /* We are asleep, so all workers better be idle. */
> > > > atomic {
> > > > i = 0;
> > > > idle = 1;
> > > > do
> > > > :: i < NUM_WORKERS ->
> > > > if
> > > > :: am_setup[i] -> idle = 0;
> > > > :: else -> skip;
> > > > fi;
> > > > i++;
> > > > :: i >= NUM_WORKERS -> break;
> > > > od;
> > > > assert(idle == 1 ||
> > > > full_sysidle_state < RCU_SYSIDLE_FULL);
> > > > }
> > > > od;
> > > > assert(full_sysidle_state <= RCU_SYSIDLE_FULL_NOTED);
> > > > od;
> > > > }
> > > >
> > > > init {
> > > > byte i = 0;
> > > >
> > > > do
> > > > :: i < NUM_WORKERS ->
> > > > am_busy[i] = 1;
> > > > am_setup[i] = 1;
> > > > run worker(i);
> > > > i++;
> > > > :: i >= NUM_WORKERS -> break;
> > > > od;
> > > > run timekeeper();
> > > > }
> > > >
> > > >
> > >
> > > --
> > > Mathieu Desnoyers
> > > EfficiOS Inc.
> > > http://www.efficios.com
> > >
> >
> >
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>
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