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From: Peter Zijlstra <peterz@infradead.org>
To: Song Liu <songliubraving@fb.com>
Cc: "open list:BPF (Safe dynamic programs and tools)" 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"acme@kernel.org" <acme@kernel.org>,
	"mingo@redhat.com" <mingo@redhat.com>,
	Kernel Team <Kernel-team@fb.com>,
	Kan Liang <kan.liang@linux.intel.com>,
	Like Xu <like.xu@linux.intel.com>,
	Alexey Budankov <alexey.budankov@linux.intel.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)
In-Reply-To: <962EDD5A-1B35-4C7F-A0A1-3EBC32EE63AB@fb.com>

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 
> pmu? 

Yep, that way you're serialized against PMIs. It's what all of the perf
core does.

> > 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
	 * handler...
	__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
stack feature.

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 :/

  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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