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From: Song Liu <songliubraving@fb.com>
To: Peter Zijlstra <peterz@infradead.org>
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: Wed, 18 Aug 2021 16:46:32 +0000	[thread overview]
Message-ID: <962EDD5A-1B35-4C7F-A0A1-3EBC32EE63AB@fb.com> (raw)
In-Reply-To: <YRzPwClswwxHXVHe@hirez.programming.kicks-ass.net>

Hi Peter,

Thanks for you quick response!

> On Aug 18, 2021, at 2:15 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Aug 17, 2021 at 06:29:37PM -0700, Song Liu wrote:
>> The typical way to access LBR is via hardware perf_event. For CPUs with
>> FREEZE_LBRS_ON_PMI support, PMI could capture reliable LBR. On the other
>> hand, LBR could also be useful in non-PMI scenario. For example, in
>> kretprobe or bpf fexit program, LBR could provide a lot of information
>> on what happened with the function.
>> In this RFC, we try to enable LBR for BPF program. This works like:
>>  1. Create a hardware perf_event with PERF_SAMPLE_BRANCH_* on each CPU;
>>  2. Call a new bpf helper (bpf_get_branch_trace) from the BPF program;
>>  3. Before calling this bpf program, the kernel stops LBR on local CPU,
>>     make a copy of LBR, and resumes LBR;
>>  4. In the bpf program, the helper access the copy from #3.
>> Please see tools/testing/selftests/bpf/[progs|prog_tests]/get_call_trace.c
>> for a detailed example. Not that, this process is far from ideal, but it
>> allows quick prototype of this feature.
>> AFAICT, the biggest challenge here is that we are now sharing LBR in PMI
>> and out of PMI, which could trigger some interesting race conditions.
>> However, if we allow some level of missed/corrupted samples, this should
>> still be very useful.
>> Please share your thoughts and comments on this. Thanks in advance!
>> +int bpf_branch_record_read(void)
>> +{
>> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>> +
>> +	intel_pmu_lbr_disable_all();
>> +	intel_pmu_lbr_read();
>> +	memcpy(this_cpu_ptr(&bpf_lbr_entries), cpuc->lbr_entries,
>> +	       sizeof(struct perf_branch_entry) * x86_pmu.lbr_nr);
>> +	*this_cpu_ptr(&bpf_lbr_cnt) = x86_pmu.lbr_nr;
>> +	intel_pmu_lbr_enable_all(false);
>> +	return 0;
>> +}
> 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 

> 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?

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?


  reply	other threads:[~2021-08-18 16:46 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 [this message]
2021-08-19 11:57     ` Peter Zijlstra
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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