Message-ID: <CAAhV-H5pfDG_tsRDL4dUYykaQ1ZwQYRDrQccpULBM5+kF4i2fA@mail.gmail.com>
Date:   Fri, 25 Aug 2023 19:15:44 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     paulmck@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Joel Fernandes <joel@...lfernandes.org>,
        Z qiang <qiang.zhang1211@...il.com>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Binbin Zhou <zhoubinbin@...ngson.cn>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

Hi, Paul,

On Fri, Aug 25, 2023 at 2:28 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Thu, Aug 24, 2023 at 11:43:04PM +0800, Huacai Chen wrote:
> > Hi, Paul,
> >
> > On Thu, Aug 24, 2023 at 9:24 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > >
> > > On Thu, Aug 24, 2023 at 08:40:00PM +0800, Huacai Chen wrote:
> > > > Hi, Paul,
> > > > On Thu, Aug 24, 2023 at 7:40 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > > On Thu, Aug 24, 2023 at 10:50:41AM +0800, Huacai Chen wrote:
> > > > > > Hi, Paul,
> > > > > > On Thu, Aug 24, 2023 at 6:41 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > > > > On Thu, Aug 24, 2023 at 12:03:25AM +0200, Thomas Gleixner wrote:
> > > > > > > > On Thu, Aug 17 2023 at 16:06, Huacai Chen wrote:
> > > > > > > > > On Thu, Aug 17, 2023 at 3:27 AM Joel Fernandes <joel@...lfernandes.org> wrote:
> > > > > > > > >> > If  do_update_jiffies_64() cannot be used in NMI context,
> > > > > > > > >>
> > > > > > > > >> Can you not make the jiffies update conditional on whether it is
> > > > > > > > >> called within NMI context?
> > > > > > > >
> > > > > > > > Which solves what? If KGDB has a breakpoint in the jiffies lock held
> > > > > > > > region then you still dead lock.
> > > > > > > >
> > > > > > > > >> I dislike that..
> > > > > > > > > Is this acceptable?
> > > > > > > > >
> > > > > > > > > void rcu_cpu_stall_reset(void)
> > > > > > > > > {
> > > > > > > > >         unsigned long delta;
> > > > > > > > >
> > > > > > > > >         delta = nsecs_to_jiffies(ktime_get_ns() - ktime_get_coarse_ns());
> > > > > > > > >
> > > > > > > > >         WRITE_ONCE(rcu_state.jiffies_stall,
> > > > > > > > >                    jiffies + delta + rcu_jiffies_till_stall_check());
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > This can update jiffies_stall without updating jiffies (but has the
> > > > > > > > > same effect).
> > > > > > > >
> > > > > > > > Now you traded the potential dead lock on jiffies lock for a potential
> > > > > > > > live lock vs. tk_core.seq. Not really an improvement, right?
> > > > > > > >
> > > > > > > > The only way you can do the above is something like the incomplete and
> > > > > > > > uncompiled below. NMI safe and therefore livelock proof time interfaces
> > > > > > > > exist for a reason.
> > > > > > >
> > > > > > > Just for completeness, another approach, with its own advantages
> > > > > > > and disadvantage, is to add something like ULONG_MAX/4 to
> > > > > > > rcu_state.jiffies_stall, but also set a counter indicating that this
> > > > > > > has been done.  Then RCU's force-quiescent processing could decrement
> > > > > > > that counter (if non-zero) and reset rcu_state.jiffies_stall when it
> > > > > > > does reach zero.
> > > > > > >
> > > > > > > Setting the counter to three should cover most cases, but "live by the
> > > > > > > heuristic, die by the heuristic".  ;-)
> > > > > > >
> > > > > > > It would be good to have some indication when gdb exited, but things
> > > > > > > like the gdb "next" command can make that "interesting" when applied to
> > > > > > > a long-running function.
> > > > > >
> > > > > > The original code is adding ULONG_MAX/2, so adding ULONG_MAX/4 may
> > > > > > make no much difference? The simplest way is adding 300*HZ, but Joel
> > > > > > dislikes that.
> > > > >
> > > > > I am not seeing the ULONG_MAX/2, so could you please point me to that
> > > > > original code?
> > > >
> > > > Maybe I misunderstand something, I say the original code means code
> > > > before commit a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall
> > > > detection in rcu_cpu_stall_reset()").
> > >
> > > Yes, my suggestion would essentially revert that patch.  It would
> > > compensate by resetting rcu_state.jiffies_stall after a few calls
> > > to rcu_gp_fqs().
> > >
> > > Alternatively, we could simply provide a way for gdb users to manually
> > > disable RCU CPU stall warnings at the beginning of their debug sessions
> > > and to manually re-enable them when they are done.
> >
> > This problem is not KGDB-specific (though it is firstly found in the
> > KGDB case), so I want to fix it in the rcu code rather than in the
> > kgdb code.
>
> Sure, for example, there is also PowerPC XMON.
>
> But this problem also is not RCU-specific.  There are also hardlockups,
> softlockups, workqueue lockups, networking timeouts, and who knows what
> all else.
>
> Plus, and again to Thomas's point, gdb breakpoints can happen anywhere.
> For example, immediately after RCU computes the RCU CPU stall time for
> a new grace period, and right before it stores it.  The gdb callout
> updates rcu_state.jiffies_stall, but that update is overwritten with a
> stale value as soon as the system starts back up.
>
> Low probabillity, to be sure, but there are quite a few places in
> the kernel right after a read from some timebase or another, and many
> (perhaps all) of these can see similar stale-time-use problems.
>
> The only way I know of to avoid these sorts of false positives is for
> the user to manually suppress all timeouts (perhaps using a kernel-boot
> parameter for your early-boot case), do the gdb work, and then unsuppress
> all stalls.  Even that won't work for networking, because the other
> system's clock will be running throughout.
>
> In other words, from what I know now, there is no perfect solution.
> Therefore, there are sharp limits to the complexity of any solution that
> I will be willing to accept.
I think the simplest solution is (I hope Joel will not angry):

void rcu_cpu_stall_reset(void)
{
        WRITE_ONCE(rcu_state.jiffies_stall, jiffies + 300*HZ);
}

300s is the upper limit of "stall timeout" we can configure
(RCU_CPU_STALL_TIMEOUT in kernel/rcu/Kconfig.debug), so it isn't just
a "magic number". In practice, 300s is also enough for any normal kgdb
operation. And compared to "resetting after a few calls to
rcu_gp_fqs()", this simple solution means "automatically resetting
after 300s".

If this is completely unacceptable, I prefer Thomas's
tick_estimate_stale_jiffies() solution.

Huacai

>
>                                                         Thanx, Paul
>
> > Huacai
> > >
> > >                                                         Thanx, Paul
> > >
> > > > Huacai
> > > > >
> > > > > The advantage of ULONG_MAX/4 over ULONG_MAX/2 is that the time_after()
> > > > > and time_before() macros have ULONG_MAX/4 slop in either direction
> > > > > before giving you the wrong answer.  You can get nearly the same result
> > > > > using ULONG_MAX/2, but it requires a bit more care.  And even on 32-bit
> > > > > HZ=1000 systems, ULONG_MAX/4 gets you more than 12 days of gdb session
> > > > > or jiffies-update delay before you start getting false positives.
> > > > >
> > > > > Then things can be reset after (say) 3 calls to rcu_gp_fqs() and
> > > > > also the current reset at the beginning of a grace period, which
> > > > > is in record_gp_stall_check_time().
> > > > >
> > > > > It would be better if RCU could get notified at both ends of the debug
> > > > > session, but given gdb commands such as "next", along with Thomas's
> > > > > point about gdb breakpoints being pretty much anywhere, this might or
> > > > > might not be so helpful in real life.  But worth looking into.
> > > > >
> > > > >                                                         Thanx, Paul
> > > > >
> > > > > > Huacai
> > > > > >
> > > > > > >
> > > > > > >                                                         Thanx, Paul
> > > > > > >
> > > > > > > > Thanks,
> > > > > > > >
> > > > > > > >         tglx
> > > > > > > > ---
> > > > > > > > --- a/kernel/time/tick-sched.c
> > > > > > > > +++ b/kernel/time/tick-sched.c
> > > > > > > > @@ -51,6 +51,13 @@ struct tick_sched *tick_get_tick_sched(i
> > > > > > > >   */
> > > > > > > >  static ktime_t last_jiffies_update;
> > > > > > > >
> > > > > > > > +unsigned long tick_estimate_stale_jiffies(void)
> > > > > > > > +{
> > > > > > > > +     ktime_t delta = ktime_get_mono_fast_ns() - READ_ONCE(last_jiffies_update);
> > > > > > > > +
> > > > > > > > +     return delta < 0 ? 0 : div_s64(delta, TICK_NSEC);
> > > > > > > > +}
> > > > > > > > +
> > > > > > > >  /*
> > > > > > > >   * Must be called with interrupts disabled !
> > > > > > > >   */
> > > > > > > >
> > > > > > > >

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