linux-kernel - Re: [PATCH v7 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch

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Date:   Fri, 10 Sep 2021 20:58:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        mingo@...hat.com, kjain@...ux.ibm.com, kernel-team@...com,
        Andrii Nakryiko <andrii@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH v7 bpf-next 3/3] selftests/bpf: add test for
 bpf_get_branch_snapshot

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.