Re: [patch V6 04/37] x86: Make hardware latency tracing explicit

[Thomas Gleixner]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> On Mon, May 18, 2020 at 10:05:56AM +0200, Thomas Gleixner wrote:
>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>> > On Sat, May 16, 2020 at 01:45:51AM +0200, Thomas Gleixner wrote:
>> >> --- a/arch/x86/kernel/nmi.c
>> >> +++ b/arch/x86/kernel/nmi.c
>> >> @@ -334,6 +334,7 @@ static noinstr void default_do_nmi(struc
>> >>  	__this_cpu_write(last_nmi_rip, regs->ip);
>> >>  
>> >>  	instrumentation_begin();
>> >> +	ftrace_nmi_handler_enter();
>> >>  
>> >>  	handled = nmi_handle(NMI_LOCAL, regs);
>> >>  	__this_cpu_add(nmi_stats.normal, handled);
>> >> @@ -420,6 +421,7 @@ static noinstr void default_do_nmi(struc
>> >>  		unknown_nmi_error(reason, regs);
>> >>  
>> >>  out:
>> >> +	ftrace_nmi_handler_exit();
>> >>  	instrumentation_end();
>> >>  }
>> >
>> > Yeah, so I'm confused about this and the previous patch too. Why not
>> > do just this? Remove that ftrace_nmi_handler.* crud from
>> > nmi_{enter,exit}() and stick it here? Why do we needs the
>> > nmi_{enter,exit}_notrace() thing?
>> 
>> Because you then have to fixup _all_ architectures which use
>> nmi_enter/exit().
>
> We probably have to anyway. But I can do that later I suppose.

Second thoughts. For #DB and #INT3 we can just keep nmi_enter(), needs
just annotation in nmi_enter() around that trace muck.

For #NMI and #MCE I rather avoid the early trace call and do it once we
have reached "stable" state, i.e. avoid it in the whole nested NMI mess.

Thanks,

        tglx