Re: DTrace syscall provider - please test/comment

[Matt Thomas <matt%3am-software.com@localhost>]

On Jun 25, 2013, at 10:19 AM, Jeff Rizzo <riz%tastylime.net@localhost> wrote:

> On 6/25/13 10:06 AM, Christos Zoulas wrote:
>> On Jun 25,  9:32am, matt%3am-software.com@localhost (Matt Thomas) wrote:
>> -- Subject: Re: DTrace syscall provider - please test/comment
>> 
>> |
>> | On Jun 25, 2013, at 5:25 AM, christos%zoulas.com@localhost (Christos 
>> Zoulas) wrote:
>> |
>> | > On Jun 24,  6:12pm, matt%3am-software.com@localhost (Matt Thomas) wrote:
>> | > -- Subject: Re: DTrace syscall provider - please test/comment
>> | >
>> | > |
>> | > | On Jun 24, 2013, at 6:01 PM, Christos Zoulas 
>> <christos%astron.com@localhost> wrote:
>> | > |
>> | > | > Can't this be done as an addition/enhancement to the trace_enter()/
>> | > | > trace_exit() facility instead of having to enter each syscall entry?
>> | > |
>> | > | that only gets called if p->p_trace_enabled is set.  So now you need
>> | > | a hook to set that on every lwp switch if the provider is tracing.
>> | >
>> | > Right, and it (dtrace) can set a different (or the same flag) to enable
>> | > it.
>> |
>> |
>> | How does it set the same flag since that's per-proc and will need to 
>> changed
>> | on context switch.
>> |
>> | A different flag is more overhead per syscall.
>> 
>> I am trying to balance that against adding of two more conditionals per
>> syscall per architecture and touching dozens of source files adding the
>> same code in each one. Perhaps the syscall_plain/syscall_fancy idea
>> was not that bad after all :-( Perhaps a different bit on the same flag.
>> If any of them is set, you call trace enter, and you clear/move the
>> bit on context switch.
>> 
>> christos
> 
> I am by no means an expert on this part of the kernel, but during the course 
> of this project, I noticed that FreeBSD seems to have made a 
> kern/subr_syscall.c which has an MI place where they put their entry/exit.  
> It seemed a lot cleaner than what we have; I can't speak to performance. Can 
> someone with more design clue than I comment on this setup?

I added an inline to <sys/syscall.h>.

int sy_invoke(const struct sysent *, struct lwp *, const void *, register_t *,
       int code);

which does the trace_enter/trace_exit dance, that can be modified to do
the dtrace dance as well.