Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 10 Sep 2021 20:58:35 +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,
        Andrii Nakryiko <andrii@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>
Subject: Re: [PATCH v7 bpf-next 3/3] selftests/bpf: add test for
 bpf_get_branch_snapshot
Message-ID: <20210910185835.GR4323@worktop.programming.kicks-ass.net>
References: <20210910183352.3151445-1-songliubraving@fb.com>
 <20210910183352.3151445-4-songliubraving@fb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210910183352.3151445-4-songliubraving@fb.com>
Precedence: bulk

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.