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From: John Fastabend <john.fastabend@gmail.com>
To: Song Liu <songliubraving@fb.com>,
	bpf@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: acme@kernel.org, peterz@infradead.org, mingo@redhat.com,
	kjain@linux.ibm.com, kernel-team@fb.com,
	Song Liu <songliubraving@fb.com>
Subject: RE: [PATCH v5 bpf-next 1/3] perf: enable branch record for software events
Date: Thu, 02 Sep 2021 13:49:43 -0700	[thread overview]
Message-ID: <613138e72048_2c56f20869@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20210902165706.2812867-2-songliubraving@fb.com>

Song Liu wrote:
> The typical way to access branch record (e.g. Intel 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. Add API
> to use branch record for software use.
> 
> Note that, when the software event triggers, it is necessary to stop the
> branch record hardware asap. Therefore, static_call is used to remove some
> branch instructions in this process.
> 
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---

[...]

>  void intel_pmu_auto_reload_read(struct perf_event *event);
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index fe156a8170aa3..4fe11f4f896b1 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -57,6 +57,7 @@ struct perf_guest_info_callbacks {
>  #include <linux/cgroup.h>
>  #include <linux/refcount.h>
>  #include <linux/security.h>
> +#include <linux/static_call.h>
>  #include <asm/local.h>
>  
>  struct perf_callchain_entry {
> @@ -1612,4 +1613,26 @@ extern void __weak arch_perf_update_userpage(struct perf_event *event,
>  extern __weak u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr);
>  #endif
>  
> +/*
> + * Snapshot branch stack on software events.
> + *
> + * Branch stack can be very useful in understanding software events. For
> + * example, when a long function, e.g. sys_perf_event_open, returns an
> + * errno, it is not obvious why the function failed. Branch stack could
> + * provide very helpful information in this type of scenarios.
> + *
> + * On software event, it is necessary to stop the hardware branch recorder
> + * fast. Otherwise, the hardware register/buffer will be flushed with
> + * entries af the triggering event. Therefore, static call is used to
              ^^
nit, af->of

> + * stop the hardware recorder.
> + */
> +
> +/*
> + * cnt is the number of entries allocated for entries.
> + * Return number of entries copied to .
> + */

A bit out of scope, but LGTM.

Acked-by: John Fastabend <john.fastabend@gmail.com>

  reply	other threads:[~2021-09-02 20:49 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-02 16:57 [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Song Liu
2021-09-02 16:57 ` [PATCH v5 bpf-next 1/3] perf: enable branch record for software events Song Liu
2021-09-02 20:49   ` John Fastabend [this message]
2021-09-03  8:02   ` Peter Zijlstra
2021-09-03 16:50     ` Song Liu
2021-09-07 18:59       ` Song Liu
2021-09-07 20:50         ` Andrii Nakryiko
2021-09-03  8:27   ` Peter Zijlstra
2021-09-03 16:45     ` Song Liu
2021-09-04 10:18       ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot Song Liu
2021-09-02 20:56   ` John Fastabend
2021-09-02 22:04     ` Song Liu
2021-09-02 22:53   ` Andrii Nakryiko
2021-09-02 23:03     ` Song Liu
2021-09-02 23:05       ` Andrii Nakryiko
2021-09-03  1:03   ` Alexei Starovoitov
2021-09-03  8:28   ` Peter Zijlstra
2021-09-03 16:58     ` Song Liu
2021-09-03  8:47   ` Peter Zijlstra
2021-09-03 17:06     ` Song Liu
2021-09-03 17:10     ` Andrii Nakryiko
2021-09-04 10:24       ` Peter Zijlstra
2021-09-04 10:55         ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch_snapshot Song Liu
2021-09-02 21:05   ` John Fastabend
2021-09-02 22:54 ` [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Andrii Nakryiko

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