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Date:   Fri, 3 Sep 2021 10:47:55 +0200
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
Message-ID: <YTHhOy1gqr44C1bI@hirez.programming.kicks-ass.net>
References: <20210902165706.2812867-1-songliubraving@fb.com>
 <20210902165706.2812867-3-songliubraving@fb.com>
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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).