From: "Paul E. McKenney" <paulmck@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, john.stultz@linaro.org,
sboyd@kernel.org, corbet@lwn.net, Mark.Rutland@arm.com,
maz@kernel.org, kernel-team@fb.com, neeraju@codeaurora.org,
ak@linux.intel.com
Subject: Re: [PATCH v7 clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable
Date: Mon, 12 Apr 2021 12:57:22 -0700 [thread overview]
Message-ID: <20210412195722.GG4510@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <87y2dnz644.ffs@nanos.tec.linutronix.de>
On Mon, Apr 12, 2021 at 08:54:03PM +0200, Thomas Gleixner wrote:
> Paul,
>
> On Mon, Apr 12 2021 at 11:20, Paul E. McKenney wrote:
> > On Mon, Apr 12, 2021 at 03:08:16PM +0200, Thomas Gleixner wrote:
> >> The reason for irqsave is again historical AFAICT and nobody bothered to
> >> clean it up. spin_lock_bh() should be sufficient to serialize against
> >> the watchdog timer, though I haven't looked at all possible scenarios.
> >
> > Though if BH is disabled, there is not so much advantage to
> > invoking it from __clocksource_watchdog_kthread(). Might as
> > well just invoke it directly from clocksource_watchdog().
> >
> >> > 2. Invoke clocksource_verify_percpu() from its original
> >> > location in clocksource_watchdog(), just before the call to
> >> > __clocksource_unstable(). This relies on the fact that
> >> > clocksource_watchdog() acquires watchdog_lock without
> >> > disabling interrupts.
> >>
> >> That should be fine, but this might cause the softirq to 'run' for a
> >> very long time which is not pretty either.
> >>
> >> Aside of that, do we really need to check _all_ online CPUs? What you
> >> are trying to figure out is whether the wreckage is CPU local or global,
> >> right?
> >>
> >> Wouldn't a shirt-sleeve approach of just querying _one_ CPU be good
> >> enough? Either the other CPU has the same wreckage, then it's global or
> >> it hasn't which points to a per CPU local issue.
> >>
> >> Sure it does not catch the case where a subset (>1) of all CPUs is
> >> affected, but I'm not seing how that really buys us anything.
> >
> > Good point! My thought is to randomly pick eight CPUs to keep the
> > duration reasonable while having a good chance of hitting "interesting"
> > CPU choices in multicore and multisocket systems.
> >
> > However, if a hard-to-reproduce problem occurred, it would be good to take
> > the hit and scan all the CPUs. Additionally, there are some workloads
> > for which the switch from TSC to HPET is fatal anyway due to increased
> > overhead. For these workloads, the full CPU scan is no additional pain.
> >
> > So I am thinking in terms of a default that probes eight randomly selected
> > CPUs without worrying about duplicates (as in there would be some chance
> > that fewer CPUs would actually be probed), but with a boot-time flag
> > that does all CPUs. I would add the (default) random selection as a
> > separate patch.
>
> You can't do without making it complex, right? Keep it simple is not an
> option for a RCU hacker it seems :)
But it was simple! It just hit all the CPUs.
However, you (quite rightly) pointed out that this simple approach had
a few shortcomings. ;-)
> > I will send a new series out later today, Pacific Time.
>
> Can you do me a favour and send it standalone and not as yet another
> reply to this existing thread maze. A trivial lore link to the previous
> version gives enough context.
Will do!
Of course, it turns out that lockdep also doesn't like waited-on
smp_call_function_single() invocations from timer handlers,
so I am currently looking at other options for dealing with that
potential use-after-free. I am starting to like the looks of "only set
CLOCK_SOURCE_VERIFY_PERCPU on statically allocated clocksource structures
and let KASAN enforce this restriction", but I have not quite given up
on making it more general.
Thanx, Paul
next prev parent reply other threads:[~2021-04-12 19:57 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-06 0:40 [PATCH RFC clocksource] Do not mark clocks unstable due to delays Paul E. McKenney
2021-01-06 0:41 ` [PATCH RFC clocksource 1/5] clocksource: Provide module parameters to inject delays in watchdog paulmck
2021-01-06 0:41 ` [PATCH RFC clocksource 2/5] clocksource: Retry clock read if long delays detected paulmck
2021-01-06 16:28 ` Rik van Riel
2021-01-06 19:53 ` Paul E. McKenney
2021-01-06 20:59 ` Rik van Riel
2021-01-06 0:41 ` [PATCH RFC clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable paulmck
2021-01-06 0:41 ` [PATCH RFC clocksource 4/5] clocksource: Provide a module parameter to fuzz per-CPU clock checking paulmck
2021-01-06 0:41 ` [PATCH RFC clocksource 5/5] clocksource: Do pairwise clock-desynchronization checking paulmck
2021-01-12 0:42 ` [PATCH v2 clocksource] Do not mark clocks unstable due to delays Paul E. McKenney
2021-01-12 0:45 ` [PATCH v2 clocksource 1/5] clocksource: Provide module parameters to inject delays in watchdog paulmck
2021-01-12 0:45 ` [PATCH v2 clocksource 2/5] clocksource: Retry clock read if long delays detected paulmck
2021-01-12 0:45 ` [PATCH v2 clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable paulmck
2021-01-12 0:45 ` [PATCH v2 clocksource 4/5] clocksource: Provide a module parameter to fuzz per-CPU clock checking paulmck
2021-01-12 0:45 ` [PATCH v2 clocksource 5/5] clocksource: Do pairwise clock-desynchronization checking paulmck
2021-02-02 17:04 ` [PATCH v3 clocksource] Do not mark clocks unstable due to delays Paul E. McKenney
2021-02-02 17:06 ` [PATCH clocksource 1/5] clocksource: Provide module parameters to inject delays in watchdog paulmck
2021-02-02 17:06 ` [PATCH clocksource 2/5] clocksource: Retry clock read if long delays detected paulmck
2021-02-02 17:06 ` [PATCH clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable paulmck
2021-02-02 17:06 ` [PATCH clocksource 4/5] clocksource: Provide a module parameter to fuzz per-CPU clock checking paulmck
2021-02-02 19:51 ` Randy Dunlap
2021-02-03 0:50 ` Paul E. McKenney
2021-02-03 1:31 ` Randy Dunlap
2021-02-03 1:40 ` Paul E. McKenney
2021-02-02 17:06 ` [PATCH clocksource 5/5] clocksource: Do pairwise clock-desynchronization checking paulmck
2021-02-17 21:28 ` [PATCH v3 clocksource] Do not mark clocks unstable due to delays Paul E. McKenney
2021-02-17 21:29 ` [PATCH clocksource 1/5] clocksource: Provide module parameters to inject delays in watchdog paulmck
2021-02-17 21:29 ` [PATCH clocksource 2/5] clocksource: Retry clock read if long delays detected paulmck
2021-02-17 21:29 ` [PATCH clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable paulmck
2021-02-17 21:29 ` [PATCH clocksource 4/5] clocksource: Provide a module parameter to fuzz per-CPU clock checking paulmck
2021-02-17 21:29 ` [PATCH clocksource 5/5] clocksource: Do pairwise clock-desynchronization checking paulmck
2021-03-04 0:49 ` [PATCH v5 clocksource] Do not mark clocks unstable due to delays for v5.13 Paul E. McKenney
2021-03-04 0:53 ` [PATCH kernel/time 1/5] clocksource: Provide module parameters to inject delays in watchdog paulmck
2021-03-04 0:53 ` [PATCH kernel/time 2/5] clocksource: Retry clock read if long delays detected paulmck
2021-03-04 0:53 ` [PATCH kernel/time 3/5] clocksource: Check per-CPU clock synchronization when marked unstable paulmck
2021-03-04 0:53 ` [PATCH kernel/time 4/5] clocksource: Provide a module parameter to fuzz per-CPU clock checking paulmck
2021-03-04 0:53 ` [PATCH kernel/time 5/5] clocksource: Do pairwise clock-desynchronization checking paulmck
2021-04-02 20:29 ` [PATCH v5 clocksource] Do not mark clocks unstable due to delays for v5.13 Paul E. McKenney
2021-04-02 20:31 ` [PATCH v6 clocksource] Do not mark clocks unstable dueclocksource: Provide module parameters to inject delays in watchdog paulmck
2021-04-02 22:22 ` Thomas Gleixner
2021-04-02 22:37 ` Paul E. McKenney
2021-04-02 22:48 ` [PATCH v7 clocksource] Do not mark clocks unstable due to delays for v5.13 Paul E. McKenney
2021-04-02 22:49 ` [PATCH v7 clocksource 1/5] clocksource: Provide module parameters to inject delays in watchdog paulmck
2021-04-02 22:49 ` [PATCH v7 clocksource 2/5] clocksource: Retry clock read if long delays detected paulmck
2021-04-10 8:41 ` Thomas Gleixner
2021-04-10 23:50 ` Paul E. McKenney
2021-04-02 22:49 ` [PATCH v7 clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable paulmck
2021-04-10 9:00 ` Thomas Gleixner
2021-04-11 0:20 ` Paul E. McKenney
2021-04-11 10:33 ` Thomas Gleixner
2021-04-11 16:46 ` Paul E. McKenney
2021-04-12 4:21 ` Paul E. McKenney
2021-04-12 13:08 ` Thomas Gleixner
2021-04-12 18:20 ` Paul E. McKenney
2021-04-12 18:54 ` Thomas Gleixner
2021-04-12 19:57 ` Paul E. McKenney [this message]
2021-04-12 20:37 ` Thomas Gleixner
2021-04-12 23:18 ` Paul E. McKenney
2021-04-13 20:49 ` Thomas Gleixner
2021-04-14 4:48 ` Paul E. McKenney
2021-04-02 22:49 ` [PATCH v7 clocksource 4/5] clocksource: Provide a module parameter to fuzz per-CPU clock checking paulmck
2021-04-02 22:49 ` [PATCH v7 clocksource 5/5] clocksource: Do pairwise clock-desynchronization checking paulmck
2021-04-10 9:04 ` Thomas Gleixner
2021-04-11 0:21 ` Paul E. McKenney
2021-04-10 8:01 ` [PATCH v7 clocksource] Do not mark clocks unstable due to delays for v5.13 Thomas Gleixner
2021-04-10 23:26 ` Paul E. McKenney
2021-04-11 10:58 ` Thomas Gleixner
2021-04-11 16:50 ` Paul E. McKenney
2021-04-02 20:31 ` [PATCH v6 clocksource] Do not mark clocks unstable dueclocksource: Retry clock read if long delays detected paulmck
2021-04-02 20:31 ` [PATCH v6 clocksource] Do not mark clocks unstable dueclocksource: Check per-CPU clock synchronization when marked unstable paulmck
2021-04-02 20:31 ` [PATCH v6 clocksource] Do not mark clocks unstable dueclocksource: Provide a module parameter to fuzz per-CPU clock checking paulmck
2021-04-02 20:31 ` [PATCH v6 clocksource] Do not mark clocks unstable dueclocksource: Do pairwise clock-desynchronization checking paulmck
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