From: Huacai Chen <chenhuacai@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Z qiang <qiang.zhang1211@gmail.com>,
paulmck@kernel.org, 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: Fri, 25 Aug 2023 00:15:10 +0800 [thread overview]
Message-ID: <CAAhV-H7L0gYZv_HgXRfF01y8j+KB7ne=8H0-KxXx4nYUyY-vxQ@mail.gmail.com> (raw)
In-Reply-To: <20230824132155.GB3810470@google.com>
Hi, Joel,
On Thu, Aug 24, 2023 at 9:22 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> Hello Thomas,
>
> 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@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.
>
> Yes, we had already discussed this that jiffies update is not possible from
> here. There are too many threads since different patch revisions were being
> reviewed in different threads.
>
> > >> 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.
>
> Yes, I had already mentioned exactly this issue here of not using an NMI-safe
> interface:
> https://lore.kernel.org/all/CAEXW_YT+uw5JodtrqjY0B2xx0J8ukF=FAB9-p5rxgWobSU2P2A@mail.gmail.com/
> I like your suggestion of using last_jiffies_update though (which as you
> mentioned needs to be explored more).
>
> There are too many threads which makes the discussion hard to follow. Huacai,
> it would be great if we can keep the discussions in the same thread (Say for
> example by passing options like --in-reply-to to "git send-email" command).
I will try my best. In the early time, not all of us were involved.
And when I think we are all ready to accept the new solution, I sent a
new patch. But unfortunately some others suggest different approaches
which make the patch subject change again and again...
Now I tend to use Thomas's method by introducing and using
tick_estimate_stale_jiffies(). But let me wait for some time before
sending patches.
Huacai
>
> thanks,
>
> - Joel
>
>
> >
> > 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 !
> > */
> >
> >
next prev parent reply other threads:[~2023-08-24 16:16 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
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 [this message]
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='CAAhV-H7L0gYZv_HgXRfF01y8j+KB7ne=8H0-KxXx4nYUyY-vxQ@mail.gmail.com' \
--to=chenhuacai@kernel.org \
--cc=boqun.feng@gmail.com \
--cc=chenhuacai@loongson.cn \
--cc=frederic@kernel.org \
--cc=jiangshanlai@gmail.com \
--cc=joel@joelfernandes.org \
--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).