Messages in this thread | | | Date | Mon, 6 Aug 2018 23:05:36 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH ftrace/core] tracing: irqsoff: Account for additional preempt_disable |
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On Sun, 5 Aug 2018 20:40:49 -0700 "Joel Fernandes (Google)" <joel@joelfernandes.org> wrote:
> Recently we tried to make the preemptirqsoff tracer to use irqsoff > tracepoint probes. However this causes issues as reported by Masami: > > [2.271078] Testing tracer preemptirqsoff: .. no entries found ..FAILED! > [2.381015] WARNING: CPU: 0 PID: 1 at /home/mhiramat/ksrc/linux/kernel/ > trace/trace.c:1512 run_tracer_selftest+0xf3/0x154 > > This is due to the tracepoint code increasing the preempt nesting count > by calling an additional preempt_disable before calling into the > preemptoff tracer which messes up the preempt_count() check in > tracer_hardirqs_off. > > To fix this, make the irqsoff tracer probes balance the additional outer > preempt_disable with a preempt_enable_notrace.
I've tested it and ensured this fixes the problem.
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
And I have a comment on this code.
> > The other way to fix this is to just use SRCU for all tracepoints. > However we can't do that because we can't use NMIs from RCU context. > > Fixes: c3bc8fd637a9 ("tracing: Centralize preemptirq tracepoints > and unify their usage") > Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU") > Reported-by: Masami Hiramatsu <mhiramat@kernel.org> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > --- > kernel/trace/trace_irqsoff.c | 26 ++++++++++++++++++++++++++ > 1 file changed, 26 insertions(+) > > diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c > index 770cd30cda40..ffbf1505d5bc 100644 > --- a/kernel/trace/trace_irqsoff.c > +++ b/kernel/trace/trace_irqsoff.c > @@ -603,14 +603,40 @@ static void irqsoff_tracer_stop(struct trace_array *tr) > */ > static void tracer_hardirqs_on(void *none, unsigned long a0, unsigned long a1) > {
To ensure this function must not be preempted even if we increment preempt count, I think you should check irq_disabled() whole this process, put below here.
if (unlikely(!irq_disabled())) return;
Since irq_disabled() will be checked in irq_trace() anyway, so no problem to return here when !irq_disabled().
> + /* > + * Tracepoint probes are expected to be called with preempt disabled, > + * We don't care about being called with preempt disabled but we need > + * to know in the future if that changes so we can remove the next > + * preempt_enable. > + */ > + WARN_ON_ONCE(!preempt_count()); > + > + /* Tracepoint probes disable preemption atleast once, account for that */
* Even we do this here, we are sure that irq is disabled in this context. * which means preemption is kept disabled.
> + preempt_enable_notrace(); > + > if (!preempt_trace() && irq_trace()) > stop_critical_timing(a0, a1); > + > + preempt_disable_notrace(); > } > > static void tracer_hardirqs_off(void *none, unsigned long a0, unsigned long a1) > {
Same here.
Any though :) ?
Thank you,
> + /* > + * Tracepoint probes are expected to be called with preempt disabled, > + * We don't care about being called with preempt disabled but we need > + * to know in the future if that changes so we can remove the next > + * preempt_enable. > + */ > + WARN_ON_ONCE(!preempt_count()); > + > + /* Tracepoint probes disable preemption atleast once, account for that */ > + preempt_enable_notrace(); > + > if (!preempt_trace() && irq_trace()) > start_critical_timing(a0, a1); > + > + preempt_disable_notrace(); > } > > static int irqsoff_tracer_init(struct trace_array *tr) > -- > 2.18.0.597.ga71716f1ad-goog >
-- Masami Hiramatsu <mhiramat@kernel.org>
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