LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Song Liu <email@example.com>
To: Peter Zijlstra <firstname.lastname@example.org>
Cc: "open list:BPF (Safe dynamic programs and tools)"
Kernel Team <Kernelemail@example.com>,
Kan Liang <firstname.lastname@example.org>,
"Like Xu" <email@example.com>,
Alexey Budankov <firstname.lastname@example.org>
Subject: Re: [RFC] bpf: lbr: enable reading LBR from tracing bpf programs
Date: Thu, 19 Aug 2021 16:46:20 +0000 [thread overview]
Message-ID: <AB509D87-67C6-4B7F-AEFB-2324845C310C@fb.com> (raw)
Thanks for these helpful information and insights!
> On Aug 19, 2021, at 4:57 AM, Peter Zijlstra <email@example.com> wrote:
> 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
> 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...
Hmm... This is a little weird to me.
IIUC, we need to call perf_inject_event() after the software event, say
a kretprobe, triggers. So it gonna look like:
1. kretprobe trigger;
2. handler calls perf_inject_event();
3. PMI kicks in, and saves LBR;
4. after the PMI, consumer of LBR uses the saved data;
However, given perf_inject_event() disables PMU, we can just save the LBR
right there? And it should be a lot easier? Something like:
1. kretprobe triggers;
2. handler calls perf_snapshot_lbr();
2.2 saves LBR
3. consumer of LBR uses the saved data;
What is the downside of this approach?
>> 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.
VLBR event does look similar to the use case we have. I will take a closer
look. Thanks for the pointer!
> 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 :/
Yeah, I guess it could be a "LBR only hardware event". All it needs to do
is to keep LBR enabled (lbr_users++). I will try to keep the arch support
next prev parent reply other threads:[~2021-08-19 16:46 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-18 1:29 Song Liu
2021-08-18 9:15 ` Peter Zijlstra
2021-08-18 16:46 ` Song Liu
2021-08-19 11:57 ` Peter Zijlstra
2021-08-19 16:46 ` Song Liu [this message]
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
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--subject='Re: [RFC] bpf: lbr: enable reading LBR from tracing bpf programs' \
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).