Messages in this thread | | | Subject | Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt_counter change | From | Daniel Bristot de Oliveira <> | Date | Tue, 4 Jun 2019 12:12:36 +0200 |
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On 31/05/2019 09:47, Joel Fernandes wrote: > On Wed, May 29, 2019 at 11:40:34AM +0200, Daniel Bristot de Oliveira wrote: >> On 29/05/2019 10:33, Peter Zijlstra wrote: >>> On Tue, May 28, 2019 at 05:16:23PM +0200, Daniel Bristot de Oliveira wrote: >>>> The preempt_disable/enable tracepoint only traces in the disable <-> enable >>>> case, which is correct. But think about this case: >>>> >>>> ---------------------------- %< ------------------------------ >>>> THREAD IRQ >>>> | | >>>> preempt_disable() { >>>> __preempt_count_add(1) >>>> -------> smp_apic_timer_interrupt() { >>>> preempt_disable() >>>> do not trace (preempt count >= 1) >>>> .... >>>> preempt_enable() >>>> do not trace (preempt count >= 1) >>>> } >>>> trace_preempt_disable(); >>>> } >>>> ---------------------------- >% ------------------------------ >>>> >>>> The tracepoint will be skipped. >>> >>> .... for the IRQ. But IRQs are not preemptible anyway, so what the >>> problem? >> >> >> right, they are. >> >> exposing my problem in a more specific way: >> >> To show in a model that an event always takes place with preemption disabled, >> but not necessarily with IRQs disabled, it is worth having the preemption >> disable events separated from IRQ disable ones. >> >> The main reason is that, although IRQs disabled postpone the execution of the >> scheduler, it is more pessimistic, as it also delays IRQs. So the more precise >> the model is, the less pessimistic the analysis will be. >> >> But there are other use-cases, for instance: >> >> (Steve, correct me if I am wrong) >> >> The preempt_tracer will not notice a "preempt disabled" section in an IRQ >> handler if the problem above happens. >> >> (Yeah, I know these problems are very specific... but...) > > I agree with the problem. I think Daniel does not want to miss the preemption > disabled event caused by the IRQ disabling.
Hi Joel!
Correct, but ... look bellow.
>>>> To avoid skipping the trace, the change in the counter should be "atomic" >>>> with the start/stop, w.r.t the interrupts. >>>> >>>> Disable interrupts while the adding/starting stopping/subtracting. >>> >>>> +static inline void preempt_add_start_latency(int val) >>>> +{ >>>> + unsigned long flags; >>>> + >>>> + raw_local_irq_save(flags); >>>> + __preempt_count_add(val); >>>> + preempt_latency_start(val); >>>> + raw_local_irq_restore(flags); >>>> +} >>> >>>> +static inline void preempt_sub_stop_latency(int val) >>>> +{ >>>> + unsigned long flags; >>>> + >>>> + raw_local_irq_save(flags); >>>> + preempt_latency_stop(val); >>>> + __preempt_count_sub(val); >>>> + raw_local_irq_restore(flags); >>>> +} >>> >>> That is hideously expensive :/ >> >> Yeah... :-( Is there another way to provide such "atomicity"? >> >> Can I use the argument "if one has these tracepoints enabled, they are not >> considering it as a hot-path?" > > The only addition here seems to the raw_local_irq_{save,restore} around the > calls to increment the preempt counter and start the latency tracking. > > Is there any performance data with the tracepoint enabled and with/without > this patch? Like with hackbench?
I discussed this with Steve at the Summit on the Summit (the reason why I did not reply this email earlier is because I was in the conf/traveling), and he also agrees with peterz, disabling and (mainly) re-enabling IRQs costs too much.
We need to find another way to resolve this problem (or mitigate the cost).... :-(.
Ideas?
Thanks!!
-- Daniel
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