linux-kernel - Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall

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Message-ID: <2681134d-cc88-49a0-a1bc-4ec0816288f6@paulmck-laptop>
Date:   Mon, 28 Aug 2023 03:47:12 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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()

On Sun, Aug 27, 2023 at 06:11:40PM -0400, Joel Fernandes wrote:
> On Sun, Aug 27, 2023 at 1:51 AM Huacai Chen <chenhuacai@...nel.org> wrote:
> [..]
> > > > > > 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?
> > If you can cc my gmail <chenhuacai@...il.com>, that could be better.
> 
> Sure, will do.
> 
> > I have read your patch, maybe the counter (nr_fqs_jiffies_stall)
> > should be atomic_t and we should use atomic operation to decrement its
> > value. Because rcu_gp_fqs() can be run concurrently, and we may miss
> > the (nr_fqs == 1) condition.
> 
> I don't think so. There is only 1 place where RMW operation happens
> and rcu_gp_fqs() is called only from the GP kthread. So a concurrent
> RMW (and hence a lost update) is not possible.

Huacai, is your concern that the gdb user might have created a script
(for example, printing a variable or two, then automatically continuing),
so that breakpoints could happen in quick successsion, such that the
second breakpoint might run concurrently with rcu_gp_fqs()?

If this can really happen, the point that Joel makes is a good one, namely
that rcu_gp_fqs() is single-threaded and (absent rcutorture) runs only
once every few jiffies.  And gdb breakpoints, even with scripting, should
also be rather rare.  So if this is an issue, a global lock should do the
trick, perhaps even one of the existing locks in the rcu_state structure.
The result should then be just as performant/scalable and a lot simpler
than use of atomics.

> Could you test the patch for the issue you are seeing and provide your
> Tested-by tag? Thanks,

Either way, testing would of course be very good!  ;-)

							Thanx, Paul