Re: [PATCH v7 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch_snapshot

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 10, 2021 at 11:33:52AM -0700, Song Liu wrote:
> +	/* Given we stop LBR in software, we will waste a few entries.
> +	 * But we should try to waste as few as possible entries. We are at
> +	 * about 7 on x86_64 systems.
> +	 * Add a check for < 10 so that we get heads-up when something
> +	 * changes and wastes too many entries.
> +	 */
> +	ASSERT_LT(skel->bss->wasted_entries, 10, "check_wasted_entries");

It might be worth pointing out that you can easily bust this limit by
enabling all the various tracepoints that are still in that code, but
that that isn't a hard error since that's not the expected use case.

For example there's the wrmsr tracepoint that will inject 6 or so
branches on top of that you now have. And I also think there's a
tracepoint in local_irq_save() that can trigger.