Re: [RFC] ftrace / perf 'recursion'

[Peter Zijlstra]

On Wed, Aug 17, 2016 at 11:19:53AM +0200, Peter Zijlstra wrote:
> 
> blergh, now with LKML added...
> 
> ---
> 
> Much like: d525211f9d1b ("perf: Fix irq_work 'tail' recursion")
> 
> I found another infinite recursion problem with irq_work:
> 
>  <IRQ>  [<ffffffff811bb985>] ? perf_output_begin_forward+0x5/0x1e0
>  [<ffffffff81067835>] ? arch_irq_work_raise+0x5/0x40
>  [<ffffffff811ba170>] ? perf_event_output_forward+0x30/0x60
>  [<ffffffff81067835>] arch_irq_work_raise+0x5/0x40
>  [<ffffffff811ab547>] irq_work_queue+0x97/0xa0
>  [<ffffffff81067835>] ? arch_irq_work_raise+0x5/0x40
>  [<ffffffff811ab547>] ? irq_work_queue+0x97/0xa0
>  [<ffffffff811af88f>] __perf_event_overflow+0xcf/0x1b0
>  [<ffffffff811afa0a>] perf_swevent_overflow+0x9a/0xc0
>  [<ffffffff811afa8d>] perf_swevent_event+0x5d/0x80
>  [<ffffffff811b0472>] perf_tp_event+0x1a2/0x1b0
>  [<ffffffff81a559b0>] ? _raw_spin_trylock+0x30/0x30
>  [<ffffffff8119dc73>] ? perf_ftrace_function_call+0x83/0xd0
>  [<ffffffff8117db25>] ? ftrace_ops_assist_func+0xb5/0x110
>  [<ffffffff8117db25>] ? ftrace_ops_assist_func+0xb5/0x110
>  [<ffffffff810df52d>] ? do_send_sig_info+0x5d/0x80
>  [<ffffffff81a559b0>] ? _raw_spin_trylock+0x30/0x30
>  [<ffffffff8119db3f>] ? perf_trace_buf_alloc+0x1f/0xa0
>  [<ffffffff8124740b>] ? kill_fasync+0x6b/0x90
>  [<ffffffff81a559b0>] ? _raw_spin_trylock+0x30/0x30
>  [<ffffffff8119dc73>] ? perf_ftrace_function_call+0x83/0xd0
>  [<ffffffff81067753>] ? smp_irq_work_interrupt+0x33/0x40
>  [<ffffffff810d6f20>] ? irq_enter+0x70/0x70
>  [<ffffffff8119dcaf>] perf_ftrace_function_call+0xbf/0xd0
>  [<ffffffff8117db25>] ? ftrace_ops_assist_func+0xb5/0x110
>  [<ffffffff8117db25>] ftrace_ops_assist_func+0xb5/0x110
>  [<ffffffff81067753>] ? smp_irq_work_interrupt+0x33/0x40
>  [<ffffffff810d6f20>] ? irq_enter+0x70/0x70
>  [<ffffffffa157e077>] 0xffffffffa157e077
>  [<ffffffff8124740b>] ? kill_fasync+0x6b/0x90
>  [<ffffffff810d6f25>] ? irq_exit+0x5/0xb0
>  [<ffffffff810d6f25>] irq_exit+0x5/0xb0
>  [<ffffffff81067753>] smp_irq_work_interrupt+0x33/0x40
>  [<ffffffff810d6f25>] ? irq_exit+0x5/0xb0
>  [<ffffffff81067753>] ? smp_irq_work_interrupt+0x33/0x40
>  [<ffffffff81a580b9>] irq_work_interrupt+0x89/0x90
>  <EOI>
> 
> Here every irq_work execution triggers another irq_work queue, which
> gets us stuck in an IRQ loop ad infinitum.
> 
> This is through function tracing of irq_exit(), but the same can be done
> through function tracing of pretty much anything else around there and
> through the explicit IRQ_WORK_VECTOR tracepoints.
> 
> The only 'solution' is something like the below, which I absolutely
> detest because it makes the irq_work code slower for everyone.
> 
> Also, this doesn't fix the problem for any other arch :/
> 
> I would much rather tag the whole irq_work thing notrace and remove the
> tracepoints, but I'm sure that'll not be a popular solution either :/

So I also found: d5b5f391d434 ("ftrace, perf: Avoid infinite event
generation loop") which is very similar.

I suppose that the entry tracepoint is harmless because you cannot queue
an irq_work that is already queued, so there it doesn't cause the
recursion.

So how to extend the same to function tracer .... we'd have to mark
exiting_irq() -> irq_exit() and everything from that as notrace, which
seems somewhat excessive, fragile and undesired because tracing those
functions is useful in other context :/