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From: Thomas Gleixner <tglx@linutronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>, paulmck <paulmck@kernel.org>,
	"Joel Fernandes\, Google" <joel@joelfernandes.org>,
	Frederic Weisbecker <frederic@kernel.org>
Subject: Re: Instrumentation and RCU
Date: Tue, 10 Mar 2020 17:48:45 +0100
Message-ID: <87imjc5f6a.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <1489283504.23399.1583852595008.JavaMail.zimbra@efficios.com>

Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:
> ----- On Mar 9, 2020, at 3:52 PM, Thomas Gleixner tglx@linutronix.de wrote:
>> In a quick test I did with a invalid syscall number with profiling the
>> trace_hardirqs_off() is pretty prominent and goes down by roughly a
>> factor of 2 when I move it past enter_from_user_mode() and use just the
>> non RCU idle variant.
>
> I think one issue here is that trace_hardirqs_off() is now shared between
> lockdep and tracing. For lockdep, we have the following comment:
>
>         /*
>          * IRQ from user mode.
>          *
>          * We need to tell lockdep that IRQs are off.  We can't do this until
>          * we fix gsbase, and we should do it before enter_from_user_mode
>          * (which can take locks).  Since TRACE_IRQS_OFF is idempotent,
>          * the simplest way to handle it is to just call it twice if
>          * we enter from user mode.  There's no reason to optimize this since
>          * TRACE_IRQS_OFF is a no-op if lockdep is off.
>          */
>         TRACE_IRQS_OFF
>
>         CALL_enter_from_user_mode
>
> 1:
>         ENTER_IRQ_STACK old_rsp=%rdi save_ret=1
>         /* We entered an interrupt context - irqs are off: */
>         TRACE_IRQS_OFF
>
> which seems to imply that lockdep requires TRACE_IRQS_OFF to be performed
> _before_ entering from usermode. I don't expect this to be useful at all for
> other tracers though. I think this should be replaced by a new e.g.
> LOCKDEP_ENTER_FROM_USER_MODE or such which would call into lockdep without
> calling other tracers.

See the entry series I'm working on. Aside of moving all this nonsense
into C-code it splits lockdep and tracing so it looks like this:

            lockdep_hardirqs_off();
            user_exit_irqsoff();
            __trace_hardirqs_off();

The latter uses regular RCU and not the scru/rcu_irq dance.

>> Right, but that still does the whole rcu_irq dance especially in the
>> entry code just to trace 50 or 100 instructions which are turning on RCU
>> anyway.
>
> Agreed. Would changing this to a lockdep-specific call as I suggest above
> solve this ?

That split exist for a few weeks now at least in my patches :)

>>> If a tracer recurses, or if a tracer attempts to trace another tracer, the
>>> instrumentation would break the recursion chain by preventing instrumentation
>>> from firing. If we end up caring about tracers tracing other tracers, we could
>>> have one distinct flag per tracer and let each tracer break the recursion chain.
>>>
>>> Having this flag per kernel stack rather than per CPU or per thread would
>>> allow tracing of nested interrupt handlers (and NMIs), but would break
>>> call chains both within the same stack or going through a trap. I think
>>> it could be a nice complementary safety net to handle mishaps in a non-fatal
>>> way.
>> 
>> That works as long as none of this uses breakpoint based patching to
>> dynamically disable/enable stuff.
>
> I'm clearly missing something here. I was expecting the "in_tracing" flag trick
> to be able to fix the breakpoint recursion issue. What is the problem I'm missing
> here ?

How do you "fix" that when you can't reach the tracepoint because you
trip over a breakpoint and then while trying to fixup that stuff you hit
another one?

Thanks,

        tglx

  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-09 17:02 Thomas Gleixner
2020-03-09 18:15 ` Steven Rostedt
2020-03-09 18:42   ` Joel Fernandes
2020-03-09 19:07     ` Steven Rostedt
2020-03-09 19:20       ` Mathieu Desnoyers
2020-03-16 15:02       ` Joel Fernandes
2020-03-09 18:59   ` Thomas Gleixner
2020-03-10  8:09     ` Masami Hiramatsu
2020-03-10 11:43       ` Thomas Gleixner
2020-03-10 15:31         ` Mathieu Desnoyers
2020-03-10 15:46           ` Steven Rostedt
2020-03-10 16:21             ` Mathieu Desnoyers
2020-03-11  0:18               ` Masami Hiramatsu
2020-03-11  0:37                 ` Mathieu Desnoyers
2020-03-11  7:48                   ` Masami Hiramatsu
2020-03-10 16:06         ` Masami Hiramatsu
2020-03-12 13:53         ` Peter Zijlstra
2020-03-10 15:24       ` Mathieu Desnoyers
2020-03-10 17:05       ` Daniel Thompson
2020-03-09 18:37 ` Mathieu Desnoyers
2020-03-09 18:44   ` Steven Rostedt
2020-03-09 18:52     ` Mathieu Desnoyers
2020-03-09 19:09       ` Steven Rostedt
2020-03-09 19:25         ` Mathieu Desnoyers
2020-03-09 19:52   ` Thomas Gleixner
2020-03-10 15:03     ` Mathieu Desnoyers
2020-03-10 16:48       ` Thomas Gleixner [this message]
2020-03-10 17:40         ` Mathieu Desnoyers
2020-03-10 18:31           ` Thomas Gleixner
2020-03-10 18:37             ` Mathieu Desnoyers
2020-03-10  1:40   ` Alexei Starovoitov
2020-03-10  8:02     ` Thomas Gleixner
2020-03-10 16:54     ` Paul E. McKenney
2020-03-17 17:56     ` Joel Fernandes
2020-03-09 20:18 ` Peter Zijlstra
2020-03-09 20:47 ` Paul E. McKenney
2020-03-09 20:58   ` Steven Rostedt
2020-03-09 21:25     ` Paul E. McKenney
2020-03-09 23:52   ` Frederic Weisbecker
2020-03-10  2:26     ` Paul E. McKenney
2020-03-10 15:13   ` Mathieu Desnoyers
2020-03-10 16:49     ` Paul E. McKenney
2020-03-10 17:22       ` Mathieu Desnoyers
2020-03-10 17:26         ` Paul E. McKenney

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