From: Peter Zijlstra <firstname.lastname@example.org>
To: Song Liu <email@example.com>
Cc: "open list:BPF (Safe dynamic programs and tools)"
Kernel Team <Kernelfirstname.lastname@example.org>,
Kan Liang <email@example.com>,
Like Xu <firstname.lastname@example.org>,
Alexey Budankov <email@example.com>
Subject: Re: [RFC] bpf: lbr: enable reading LBR from tracing bpf programs
Date: Thu, 19 Aug 2021 13:57:26 +0200 [thread overview]
Message-ID: <YR5HJkPyaM3TWkkl@hirez.programming.kicks-ass.net> (raw)
On Wed, Aug 18, 2021 at 04:46:32PM +0000, Song Liu wrote:
> > Urgghhh.. I so really hate BPF specials like this.
> I don't really like this design either. But it does show that LBR can be
> very useful in non-PMI scenario.
> > Also, the PMI race
> > you describe is because you're doing abysmal layer violations. If you'd
> > have used perf_pmu_disable() that wouldn't have been a problem.
> Do you mean instead of disable/enable lbr, we disable/enable the whole
Yep, that way you're serialized against PMIs. It's what all of the perf
> > I'd much rather see a generic 'fake/inject' PMI facility, something that
> > works across the board and isn't tied to x86/intel.
> How would that work? Do we have a function to trigger PMI from software,
> and then gather the LBR data after the PMI? This does sound like a much
> cleaner solution. Where can I find code examples that fake/inject PMI?
We don't yet have anything like it; but it would look a little like:
void perf_inject_event(struct perf_event *event, struct pt_regs *regs)
struct perf_sample_data data;
struct pmu *pmu = event->pmu;
unsigned long flags;
perf_sample_data_init(&data, 0, 0);
* XXX or a variant with more _ that starts at the overflow
__perf_event_overflow(event, 0, &data, regs);
But please consider carefully, I haven't...
> There is another limitation right now: we need to enable LBR with a
> hardware perf event (cycles, etc.). However, unless we use the event for
> something else, it wastes a hardware counter. So I was thinking to allow
> software event, i.e. dummy event, to enable LBR. Does this idea sound
> sane to you?
We have a VLBR dummy event, but I'm not sure it does exactly as you
want. However, we should also consider Power, which also has the branch
You can't really make a software event with LBR on, because then it
wouldn't be a software event anymore. You'll need some hybrid like
thing, which will be yuck and I suspect it needs arch support one way or
the other :/
next prev parent reply other threads:[~2021-08-19 11:58 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-18 1:29 [RFC] bpf: lbr: enable reading LBR from tracing bpf programs Song Liu
2021-08-18 9:15 ` Peter Zijlstra
2021-08-18 16:46 ` Song Liu
2021-08-19 11:57 ` Peter Zijlstra [this message]
2021-08-19 16:46 ` Song Liu
2021-08-19 18:06 ` Peter Zijlstra
2021-08-19 18:22 ` Song Liu
2021-08-19 18:27 ` Peter Zijlstra
2021-08-19 18:45 ` Song Liu
2021-08-20 7:33 ` Song Liu
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