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:
>> Do LBR, PEBS, and similar report user regs or do they merely want to
>> know the instruction format? If the latter, I could whip up a tiny
>> function to do just that (like perf_get_regs_user but just for ABI --
>> it would be simpler).
>
> Just the instruction format, nothing else.
>
>> >> 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? 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.
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?
I haven't dug in to the LBR code much.
>>
>> 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?
>
> Also, HSW+ PEBS doesn't do the fixup anymore.
>
>> If I'm understanding this right (a big, big if), if we get a PEBS
>> overflow while running in user mode, we'll dump out the user regs (and
>> call perf_get_regs_user) and all the PEBS entries (subject to
>> exclude_kernel and with all the decoding magic). So, in that case, we
>> call perf_get_user_regs.
>
> We only dump user regs if PERF_SAMPLE_REGS_USER, and in case we hit
> userspace userspace with the interrupt we use the interrupt regs; see
> perf_sample_regs_user().
>
>> If we get a PEBS overflow while running in kernel mode, we'll report
>> the kernel regs (if !exclude_kernel) and report the PEBS data as well.
>> If any of those records are in user mode, then, ideally, we'd invoke
>> perf_get_regs_user or similar *once* to get the ABI. Although, if we
>> can get the user ABI efficiently enough, then maybe we don't care if
>> we call it once per PEBS record.
>
> Right, if we interrupt kernel mode, we'll call perf_get_regs_user() if
> PERF_SAMPLE_REGS_USER (| PERF_SAMPLE_STACK_USER).
But not in get_perf_callchain. So we'll show the correct user *regs*
but not the current user callchain under some conditions, AFAICS.
>
> 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.
--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kselftest" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
