Re: [Qemu-devel] [PATCHv1 0/4] Timers: add timer debugging through -timer-debug-log

[Alex Bligh]

Paolo,

On 26 Oct 2013, at 08:18, Paolo Bonzini wrote:

> The simple trace backend includes a nanosecond value that is the same as
> rt_clock, so you can correlate the last timer_mod with the next
> iteration in timerlist_run_timers (I would put the tracepoint inside the
> loop, just before the callback is done).  Similarly, in systemtap you
> can just use the builtin gettimeofday_ns() function
> 
> All clocks are basically in nanoseconds (the only difference between
> vm_clock and rt_clock is that the former stops when the VM is stopped),
> so you can get the average expiry delta from there.
> 
> Of course it requires some postprocessing or a systemtap script, but we
> can put that into scripts/ or give it to the user.  See
> https://sourceware.org/ml/systemtap/2011-q3/txt00003.txt for an example,
> I suppose the "expiry time" computation would look like this:

OK I need to look at the tracepoint stuff a bit more it seems :-)

If we can do this without a patch (save to put some tracepoints in),
that's obviously better than doing it with a patch. I don't like
the file particularly either.

> global modtime, expirytot, firedcnt
> 
> probe qemu.system.x86_64.timer_mod {
>   loc = sprintf("%s:%d", timer_file, timer_line)

How do timer_file and timer_line get filled in? If those still
need to be put in through code changes, how about taking
patch 1 of the series (that fills in timer->dbg with this
information), save for the unnecessary additions to the structure,
plus whatever is necessary to put the tracepoints in?

Alex

>   modtime[pid(), loc] = gettimeofday_ns()
> }
> probe qemu.system.x86_64.timer_run {
>   loc = sprintf("%s:%d", timer_file, timer_line)
>   expirytot[pid(), loc] += gettimeofday_ns() - modtime[pid(), loc]
>   firedcnt[pid(), loc]++
> }
> 
> probe end
> {
>  printf("\n%8s %28s %8s %8s\n", "pid", "loc", "avg expiry", "cnt")
>  foreach([p+, loc] in firedcnt) {
>    printf("%8d %28s %8d %8d\n", p, loc,
>        expirytot[p, loc] / firedcnt[p, loc],
>        firedcnt[p, loc])
>  }
> }

-- 
Alex Bligh