On Tue, 2008-03-25 at 14:03 +0000, Jon Haslam wrote:
> Hi Roman,
>
> > Here's the problem I'm facing: I need to sample a particular
> > attribute of all the LWPs in a given process. Now, sampling
> > itself is no problem -- I can use profile/tick provider and
> > all is good. The problem is: I can't seem to think of a
> > DTrace-friendly way to iterate over all the LWPs and report
> > the value of the attribute I'm interested in.
> >
> > Any suggestions?
>
> To be honest, I'm not completely sure what you're after
Well, I guess that makes you a mind reader, 'cause the rest
of your reply seems to be exactly what I was looking for. ;-)
Now, as far as implementation goes, I still have a few
questions:
> self->pidp = `pidhash[$1 & (`pid_hashsz - 1)];
what is backtick doing here in front of pidhash? Is it a way
of accessing an arbitrary kernel variable?
> BEGIN
> /self->pidp->pid_id == $1/
Wow! Could you, please, elaborate on why exactly
the predicate is needed here? My reading of the first
BEGIN statement seems to suggest that the following
will always be true:
self->pidp->pid_id == $1
Or is it just a safety measure that prevents us from
getting garbage from pidhash?
> {
> this->slot = (*(uint32_t *)self->pidp) >> 8;
> procp = `procdir[this->slot].pe_proc;
> procname = stringof(procp->p_user.u_comm);
> t = procp->p_tlist;
Wow! That's some powerful kernel magic, if you ask me. ;-)
> tick-50ms
> /pidp && t != NULL/
> {
> printf("%s lwps %d/thr (%d): %d syscalls\n", procname,
> procp->p_lwpcnt,
> t->t_tid, t->t_lwp->lwp_ru.sysc);
> t = t->t_forw;
Now, here comes the crucial question: AFAIK, p_tlist points to
a circular list of kernel threads. We are traversing this list
using t = t->t_forw. Now, what happens if 't' points to
a member of the list that used to be valid but has been
deallocated in between the two ticks of tick-50ms?
Thanks,
Roman.
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