On Thu, Apr 21, 2016 at 12:39:42PM -0700, Andy Lutomirski wrote:
> On Wed, Apr 20, 2016 at 12:05 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Wed, Apr 20, 2016 at 08:40:23AM -0700, Andy Lutomirski wrote:
> >> >> Peter, I got lost in the code that calls this. Are regs coming from
> >> >> the overflow interrupt's regs, current_pt_regs(), or
> >> >> perf_get_regs_user?
> >> >
> >> > So get_perf_callchain() will get regs from:
> >> >
> >> > - interrupt/NMI regs
> >> > - perf_arch_fetch_caller_regs()
> >> >
> >> > And when user && !user_mode(), we'll use:
> >> >
> >> > - task_pt_regs() (which arguably should maybe be perf_get_regs_user())
> >>
> >> Could you point me to this bit of the code?
> >
> > kernel/events/callchain.c:198
>
> But that only applies to the callchain code, right?
Yes, which is what I thought you were after..
> AFAICS the PEBS
> code is invoked through the x86_pmu NMI handler and always gets the
> IRQ regs. Except for this case:
>
> static inline void intel_pmu_drain_pebs_buffer(void)
> {
> struct pt_regs regs;
>
> x86_pmu.drain_pebs(®s);
> }
>
> which seems a bit confused.
Yes, so that only gets used with 'large' pebs, which requires no other
flags than PERF_FRERERUNNING_FLAGS, which precludes the regs set from
being used.
Could definitely use a comment.
> I don't suppose we could arrange to pass something consistent into the
> PEBS handlers...
>
> Or is the PEBS code being called from the callchain code somehow?
No. I think we were/are slightly talking past one another.
> >> One call to perf_get_user_regs per interrupt shouldn't be too bad --
> >> certainly much better then one per PEBS record. One call to get user
> >> ABI per overflow would be even less bad, but at that point, folding it
> >> in to the PEBS code wouldn't be so bad either.
> >
> > Right; although note that the whole fixup_ip() thing requires a single
> > record per interrupt (for we need the LBR state for each record in order
> > to rewind).
>
> So do earlier PEBS events not get rewound? Or so we just program the
> thing to only ever give us one event at a time?
The latter; we program PEBS such that it can hold but a single record
and thereby assure we get an interrupt for each record.
> > The problem here is that the overflow stuff is designed for a single
> > 'event' per interrupt, so passing it data for multiple events is
> > somewhat icky.
>
> It also seems that there's a certain amount of confusion as to exactly
> what "regs" means in various contexts. Or at least I'm confused by
> it.
Yes, there's too much regs.
Typically 'regs' is the 'interrrupt'/'event' regs, that is the register
set at eventing time. For sampling hardware PMUs this is NMI/IRQ like
things, for software events this ends up being
perf_arch_fetch_caller_regs().
Then there's PERF_SAMPLE_REGS_USER|PERF_SAMPLE_STACK_USER, which, for
each event with it set, use perf_get_regs_user() to dump the thing into
our ringbuffer as part of the event record.
And then there's the callchain code, which first unwinds kernel space if
the 'interrupt'/'event' reg set points into the kernel, and then uses
task_pt_regs() (which I think we agree should be perf_get_regs_user())
to obtain the user regs to continue with the user stack unwind.
Finally there's PERF_SAMPLE_REGS_INTR, which dumps whatever
'interrupt/event' regs we get into the ringbuffer sample record.
Did that help? Or did I confuse you moar?
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