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
Date: Fri, 10 Sep 2021 20:58:35 +0200 [thread overview]
Message-ID: <20210910185835.GR4323@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20210910183352.3151445-4-songliubraving@fb.com>
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.
next prev parent reply other threads:[~2021-09-10 18:59 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-10 18:33 [PATCH v7 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Song Liu
2021-09-10 18:33 ` [PATCH v7 bpf-next 1/3] perf: enable branch record for software events Song Liu
2021-09-10 19:28 ` Peter Zijlstra
2021-09-10 18:33 ` [PATCH v7 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot Song Liu
2021-09-10 18:33 ` [PATCH v7 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch_snapshot Song Liu
2021-09-10 18:58 ` Peter Zijlstra [this message]
2021-09-10 19:04 ` Song Liu
2021-09-10 19:22 ` [PATCH v7 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Song Liu
2021-09-13 17:59 ` Alexei Starovoitov
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