From: Joel Fernandes <joel@joelfernandes.org>
To: Huacai Chen <chenhuacai@kernel.org>
Cc: paulmck@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
Z qiang <qiang.zhang1211@gmail.com>,
Huacai Chen <chenhuacai@loongson.cn>,
Frederic Weisbecker <frederic@kernel.org>,
Neeraj Upadhyay <quic_neeraju@quicinc.com>,
Josh Triplett <josh@joshtriplett.org>,
Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@kernel.org>,
John Stultz <jstultz@google.com>, Stephen Boyd <sboyd@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Sergey Senozhatsky <senozhatsky@chromium.org>,
rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org, Binbin Zhou <zhoubinbin@loongson.cn>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()
Date: Sat, 26 Aug 2023 23:27:08 -0400 [thread overview]
Message-ID: <CAEXW_YSock304V471X_A7WrxCWtHJGx3APmSy0k7Lc0o69D9Hg@mail.gmail.com> (raw)
In-Reply-To: <20230825232807.GA97898@google.com>
On Fri, Aug 25, 2023 at 7:28 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Fri, Aug 25, 2023 at 07:15:44PM +0800, Huacai Chen wrote:
> > Hi, Paul,
> >
> > On Fri, Aug 25, 2023 at 2:28 AM Paul E. McKenney <paulmck@kernel.org> wrote:
[..]
> > > > > > > > > > > On Thu, Aug 17, 2023 at 3:27 AM Joel Fernandes <joel@joelfernandes.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):
>
> Not angry at all, just want to help. ;-). The problem is the 300*HZ solution
> will also effect the VM workloads which also do a similar reset. Allow me few
> days to see if I can take a shot at fixing it slightly differently. I am
> trying Paul's idea of setting jiffies at a later time. I think it is doable.
> I think the advantage of doing this is it will make stall detection more
> robust in this face of these gaps in jiffie update. And that solution does
> not even need us to rely on ktime (and all the issues that come with that).
>
I wrote a patch similar to Paul's idea and sent it out for review, the
advantage being it purely is based on jiffies. Could you try it out
and let me know?
thanks,
- Joel
next prev parent reply other threads:[~2023-08-27 3:28 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-14 2:00 [PATCH V4 1/2] tick: Rename tick_do_update_jiffies64() and allow external usage Huacai Chen
2023-08-14 2:00 ` [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset() Huacai Chen
2023-08-14 16:15 ` Paul E. McKenney
2023-08-15 6:05 ` Huacai Chen
2023-08-16 3:16 ` Z qiang
2023-08-16 4:53 ` Huacai Chen
2023-08-16 5:09 ` Z qiang
2023-08-16 9:33 ` Huacai Chen
2023-08-16 10:06 ` Z qiang
2023-08-16 12:28 ` Huacai Chen
2023-08-16 15:56 ` Alan Huang
2023-08-16 16:13 ` Huacai Chen
2023-08-16 16:52 ` Alan Huang
2023-08-17 4:04 ` Huacai Chen
2023-08-23 21:41 ` Thomas Gleixner
2023-08-16 19:27 ` Joel Fernandes
2023-08-17 8:06 ` Huacai Chen
2023-08-23 22:03 ` Thomas Gleixner
2023-08-23 22:41 ` Paul E. McKenney
2023-08-24 2:50 ` Huacai Chen
2023-08-24 11:40 ` Paul E. McKenney
2023-08-24 12:40 ` Huacai Chen
2023-08-24 13:24 ` Paul E. McKenney
2023-08-24 15:43 ` Huacai Chen
2023-08-24 18:28 ` Paul E. McKenney
2023-08-25 11:15 ` Huacai Chen
2023-08-25 23:28 ` Joel Fernandes
2023-08-27 3:27 ` Joel Fernandes [this message]
2023-08-27 5:50 ` Huacai Chen
2023-08-27 22:11 ` Joel Fernandes
2023-08-28 10:47 ` Paul E. McKenney
2023-08-28 11:30 ` Huacai Chen
2023-08-28 11:54 ` Paul E. McKenney
2023-08-28 13:33 ` Joel Fernandes
2023-08-28 14:02 ` Paul E. McKenney
2023-08-28 14:37 ` Huacai Chen
2023-08-28 14:50 ` Joel Fernandes
2023-08-28 15:12 ` Huacai Chen
2023-08-28 20:47 ` Joel Fernandes
2023-08-29 4:07 ` Huacai Chen
2023-08-29 14:46 ` Joel Fernandes
2023-08-30 4:25 ` Huacai Chen
2023-08-30 10:18 ` Joel Fernandes
2023-08-30 1:04 ` Joel Fernandes
2023-08-26 1:45 ` Paul E. McKenney
2023-08-24 13:09 ` Joel Fernandes
2023-08-24 13:28 ` Paul E. McKenney
2023-08-24 16:03 ` Huacai Chen
2023-08-24 16:32 ` Huacai Chen
2023-08-24 16:34 ` Paul E. McKenney
2023-08-24 2:47 ` Huacai Chen
2023-08-24 9:39 ` Thomas Gleixner
2023-08-24 13:21 ` Joel Fernandes
2023-08-24 13:29 ` Paul E. McKenney
2023-08-24 16:15 ` Huacai Chen
2023-08-23 21:36 ` Thomas Gleixner
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAEXW_YSock304V471X_A7WrxCWtHJGx3APmSy0k7Lc0o69D9Hg@mail.gmail.com \
--to=joel@joelfernandes.org \
--cc=boqun.feng@gmail.com \
--cc=chenhuacai@kernel.org \
--cc=chenhuacai@loongson.cn \
--cc=frederic@kernel.org \
--cc=jiangshanlai@gmail.com \
--cc=josh@joshtriplett.org \
--cc=jstultz@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=mingo@kernel.org \
--cc=paulmck@kernel.org \
--cc=qiang.zhang1211@gmail.com \
--cc=quic_neeraju@quicinc.com \
--cc=rcu@vger.kernel.org \
--cc=rostedt@goodmis.org \
--cc=sboyd@kernel.org \
--cc=senozhatsky@chromium.org \
--cc=stable@vger.kernel.org \
--cc=tglx@linutronix.de \
--cc=zhoubinbin@loongson.cn \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).