From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Jiri Kosina <jkosina@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Michal Hocko <mhocko@suse.cz>, Jan Kara <jack@suse.cz>,
Frederic Weisbecker <fweisbec@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Dave Anderson <anderson@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Petr Mladek <pmladek@suse.cz>, Kay Sievers <kay@vrfy.org>
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context
Date: Wed, 18 Jun 2014 14:20:22 -0700 [thread overview]
Message-ID: <20140618212022.GV4669@linux.vnet.ibm.com> (raw)
In-Reply-To: <alpine.LNX.2.00.1406182310070.2303@pobox.suse.cz>
On Wed, Jun 18, 2014 at 11:12:48PM +0200, Jiri Kosina wrote:
> On Wed, 18 Jun 2014, Paul E. McKenney wrote:
>
> > > > /* Complain about tasks blocking the grace period. */
> > > > @@ -1044,8 +1041,7 @@ static void print_cpu_stall(struct rcu_state *rsp)
> > > > pr_cont(" (t=%lu jiffies g=%ld c=%ld q=%lu)\n",
> > > > jiffies - rsp->gp_start,
> > > > (long)rsp->gpnum, (long)rsp->completed, totqlen);
> > > > - if (!trigger_all_cpu_backtrace())
> > > > - dump_stack();
> > > > + rcu_dump_cpu_stacks(rsp);
> > >
> > > This is prone to producing not really consistent stacktraces though,
> > > right? As the target task is still running at the time the stack is being
> > > walked, it might produce stacktraces that are potentially nonsensial.
> >
> > If a CPU is stuck, the stack trace down to where it is stuck is
> > likely to be static. But yes, there is some potential for confusion.
> > My (admittedly limited) rcutorture testing produced sensible stack traces,
> > but things might be a bit uglier in other situations.
>
> I agree that it might work nicely for RCU stall detector indeed. I was
> looking for solution that'd work nicely both for RCU and for sysrq-l
> (where we can't rely on processess being stuck in any way).
Agreed. And if some more generally useful approach appears, I will be
quite happy to adjust RCU to use it. In the meantime, I expect that
my patch will be helpful.
Thanx, Paul
> > > How about sending NMI to the target CPU, so that the task is actually
> > > stopped, but printing its stacktrace from the CPU that detected the stall
> > > while it's stopped?
> > >
> > > That way, there is no printk()-from-NMI, but also the stacktrace is
> > > guaranteed to be self-consistent.
> >
> > I believe that this was what Steven was suggesting, though by using
> > tracing.
>
> My understanding was that Steven is suggesting using trace_printk() from
> NMI.
>
> > Of course, if my current approach isn't up to the job, then something
> > like this general approach would look quite good.
>
> Thanks,
>
> --
> Jiri Kosina
> SUSE Labs
>
next prev parent reply other threads:[~2014-06-18 21:20 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-09 9:10 [RFC PATCH 00/11] printk: safe printing in NMI context Petr Mladek
2014-05-09 9:10 ` [RFC PATCH 01/11] printk: rename struct printk_log to printk_msg Petr Mladek
2014-05-09 9:10 ` [RFC PATCH 02/11] printk: allow to handle more log buffers Petr Mladek
2014-05-09 9:10 ` [RFC PATCH 03/11] printk: rename "logbuf_lock" to "main_logbuf_lock" Petr Mladek
2014-05-09 9:10 ` [RFC PATCH 04/11] printk: add NMI ring and cont buffers Petr Mladek
2014-05-09 9:10 ` [RFC PATCH 05/11] printk: allow to modify NMI log buffer size using boot parameter Petr Mladek
2014-05-09 9:11 ` [RFC PATCH 06/11] printk: NMI safe printk Petr Mladek
2014-05-09 9:11 ` [RFC PATCH 07/11] printk: right ordering of the cont buffers from NMI context Petr Mladek
2014-05-09 9:11 ` [RFC PATCH 08/11] printk: try hard to print Oops message in " Petr Mladek
2014-05-09 9:11 ` [RFC PATCH 09/11] printk: merge and flush NMI buffer predictably via IRQ work Petr Mladek
2014-05-09 9:11 ` [RFC PATCH 10/11] printk: survive rotation of sequence numbers Petr Mladek
2014-05-09 9:11 ` [RFC PATCH 11/11] printk: avoid staling when merging NMI log buffer Petr Mladek
2014-05-28 22:02 ` [RFC PATCH 00/11] printk: safe printing in NMI context Jiri Kosina
2014-05-29 0:09 ` Frederic Weisbecker
2014-05-29 8:09 ` Jiri Kosina
2014-06-10 16:46 ` Frederic Weisbecker
2014-06-10 16:57 ` Linus Torvalds
2014-06-10 17:32 ` Jiri Kosina
2014-06-11 9:01 ` Petr Mládek
2014-06-18 11:03 ` Jiri Kosina
2014-06-18 14:36 ` Paul E. McKenney
2014-06-18 14:41 ` Jiri Kosina
2014-06-18 14:44 ` Paul E. McKenney
2014-06-18 14:53 ` Jiri Kosina
2014-06-18 15:07 ` Paul E. McKenney
[not found] ` <CA+55aFwPgDC6gSEPfu3i-pA4f0ZbsTSvykxzX4sXMeLbdXuKrw@mail.gmail.com>
2014-06-18 16:21 ` Paul E. McKenney
2014-06-18 16:38 ` Steven Rostedt
2014-06-18 16:43 ` Paul E. McKenney
2014-06-18 20:36 ` Jiri Kosina
2014-06-18 21:07 ` Paul E. McKenney
2014-06-18 21:12 ` Jiri Kosina
2014-06-18 21:20 ` Paul E. McKenney [this message]
2014-06-18 21:32 ` Jiri Kosina
2014-06-18 21:37 ` Paul E. McKenney
2014-06-18 23:20 ` Steven Rostedt
2014-05-30 8:13 ` Jan Kara
2014-05-30 10:10 ` Jiri Kosina
2014-06-10 16:49 ` Frederic Weisbecker
2014-06-12 11:50 ` Petr Mládek
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