From: Frederic Weisbecker <fweisbec@gmail.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Steven Rostedt <rostedt@goodmis.org>,
laijs@cn.fujitsu.com
Subject: Re: [PATCH 2/2] rcu: Keep gpnum and completed fields synchronized
Date: Sat, 11 Dec 2010 01:00:39 +0100 [thread overview]
Message-ID: <20101211000036.GD1713@nowhere> (raw)
In-Reply-To: <20101210230200.GK2125@linux.vnet.ibm.com>
On Fri, Dec 10, 2010 at 03:02:00PM -0800, Paul E. McKenney wrote:
> On Fri, Dec 10, 2010 at 10:11:11PM +0100, Frederic Weisbecker wrote:
> > When a CPU that was in an extended quiescent state wakes
> > up and catches up with grace periods that remote CPUs
> > completed on its behalf, we update the completed field
> > but not the gpnum that keeps a stale value of a backward
> > grace period ID.
> >
> > Later, note_new_gpnum() will interpret the shift between
> > the local CPU and the node grace period ID as some new grace
> > period to handle and will then start to hunt quiescent state.
> >
> > But if every grace periods have already been completed, this
> > interpretation becomes broken. And we'll be stuck in clusters
> > of spurious softirqs because rcu_report_qs_rdp() will make
> > this broken state run into infinite loop.
> >
> > The solution, as suggested by Lai Jiangshan, is to ensure that
> > the gpnum and completed fields are well synchronized when we catch
> > up with completed grace periods on their behalf by other cpus.
> > This way we won't start noting spurious new grace periods.
>
> Also good, queued!
>
> One issue -- this approach is vulnerable to overflow. I therefore
> followed up with a patch that changes the condition to
>
> if (ULONG_CMP_LT(rdp->gpnum, rdp->completed))
>
> And I clearly need to make RCU defend itself against the scenario where
> a CPU stays in dyntick-idle mode long enough for the grace-period number
> to wrap halfway around its range of possible values. Not a problem at
> the moment, and never will be for 64-bit systems, but...
>
> I will fix that up.
Oh you're right of course. I did not think about possible overflows.
Now looking at ULONG_CMP_LT() definition, if it wraps more than halfways
we are screwed anyway. I suspect it won't ever happen, but it can. Perhaps
we need some watchguard code in note_new_gpnum() to fixup that corner case.
>
> Thanx, Paul
>
> > Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@elte.hu>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Steven Rostedt <rostedt@goodmis.org
> > ---
> > kernel/rcutree.c | 9 +++++++++
> > 1 files changed, 9 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 8c4ed60..2e16da3 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -683,6 +683,15 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
> > rdp->completed = rnp->completed;
> >
> > /*
> > + * If we were in an extended quiescent state, we may have
> > + * missed some grace periods that others CPUs took care on
> > + * our behalf. Catch up with this state to avoid noting
> > + * spurious new grace periods.
> > + */
> > + if (rdp->completed > rdp->gpnum)
> > + rdp->gpnum = rdp->completed;
> > +
> > + /*
> > * If another CPU handled our extended quiescent states and
> > * we have no more grace period to complete yet, then stop
> > * chasing quiescent states.
> > --
> > 1.7.3.2
> >
next prev parent reply other threads:[~2010-12-11 0:00 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-12-10 21:11 [PATCH 0/2 v2] rcu: Fix series of spurious RCU softirqs Frederic Weisbecker
2010-12-10 21:11 ` [PATCH 1/2] rcu: Stop chasing QS if another CPU did it for us Frederic Weisbecker
2010-12-10 22:58 ` Paul E. McKenney
2010-12-10 23:33 ` Frederic Weisbecker
2010-12-10 21:14 ` [PATCH 0/2 v2] rcu: Fix series of spurious RCU softirqs Frederic Weisbecker
[not found] ` <1292015471-19227-3-git-send-email-fweisbec@gmail.com>
2010-12-10 23:02 ` [PATCH 2/2] rcu: Keep gpnum and completed fields synchronized Paul E. McKenney
2010-12-10 23:39 ` Paul E. McKenney
2010-12-10 23:47 ` Frederic Weisbecker
2010-12-11 0:04 ` Paul E. McKenney
2010-12-11 0:15 ` Frederic Weisbecker
2010-12-11 0:58 ` Paul E. McKenney
2010-12-11 1:21 ` Frederic Weisbecker
2010-12-11 6:36 ` Paul E. McKenney
2010-12-11 0:00 ` Frederic Weisbecker [this message]
2010-12-11 0:48 ` Paul E. McKenney
2010-12-11 0:51 ` Frederic Weisbecker
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=20101211000036.GD1713@nowhere \
--to=fweisbec@gmail.com \
--cc=a.p.zijlstra@chello.nl \
--cc=laijs@cn.fujitsu.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@elte.hu \
--cc=paulmck@linux.vnet.ibm.com \
--cc=rostedt@goodmis.org \
--cc=tglx@linutronix.de \
/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).