From: Peter Zijlstra <peterz@infradead.org>
To: Song Liu <songliubraving@fb.com>
Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
acme@kernel.org, mingo@redhat.com, kjain@linux.ibm.com,
kernel-team@fb.com
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot
Date: Fri, 3 Sep 2021 10:47:55 +0200 [thread overview]
Message-ID: <YTHhOy1gqr44C1bI@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210902165706.2812867-3-songliubraving@fb.com>
On Thu, Sep 02, 2021 at 09:57:05AM -0700, Song Liu wrote:
> +BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags)
> +{
> + static const u32 br_entry_size = sizeof(struct perf_branch_entry);
> + u32 entry_cnt = size / br_entry_size;
> +
> + if (unlikely(flags))
> + return -EINVAL;
> +
> + if (!buf || (size % br_entry_size != 0))
> + return -EINVAL;
> +
> + entry_cnt = static_call(perf_snapshot_branch_stack)(buf, entry_cnt);
That's at least 2, possibly 3 branches just from the sanity checks, plus
at least one from starting the BPF prog and one from calling this
function, gets you at ~5 branch entries gone before you even do the
snapshot thing.
Less if you're in branch-stack mode.
Can't the validator help with getting rid of the some of that?
I suppose you have to have this helper function because the JIT cannot
emit static_call()... although in this case one could cheat and simply
emit a call to static_call_query() and not bother with dynamic updates
(because there aren't any).
next prev parent reply other threads:[~2021-09-03 8:48 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
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 [this message]
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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