From: Peter Zijlstra <peterz@infradead.org>
To: Tao Chen <chen.dylane@linux.dev>
Cc: mingo@redhat.com, acme@kernel.org, namhyung@kernel.org,
mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com,
kan.liang@linux.intel.com, song@kernel.org, ast@kernel.org,
daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev,
eddyz87@gmail.com, yonghong.song@linux.dev,
john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me,
haoluo@google.com, linux-perf-users@vger.kernel.org,
linux-kernel@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH bpf-next v3 2/2] bpf: Use per-cpu BPF callchain entry to save callchain
Date: Mon, 20 Oct 2025 13:03:03 +0200 [thread overview]
Message-ID: <20251020110303.GS3419281@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20251019170118.2955346-3-chen.dylane@linux.dev>
On Mon, Oct 20, 2025 at 01:01:18AM +0800, Tao Chen wrote:
> As Alexei noted, get_perf_callchain() return values may be reused
> if a task is preempted after the BPF program enters migrate disable
> mode. Drawing on the per-cpu design of bpf_bprintf_buffers,
> per-cpu BPF callchain entry is used here.
And now you can only unwind 3 tasks, and then start failing. This is
acceptable, why?
> - if (may_fault)
> - rcu_read_lock(); /* need RCU for perf's callchain below */
> -
I know you propose to remove this code; but how was that correct? The
perf callchain code hard relies on non-preemptible context, RCU does not
imply such a thing.
> if (trace_in)
> trace = trace_in;
> - else if (kernel && task)
> trace = get_callchain_entry_for_task(task, max_depth);
> - else
> - trace = get_perf_callchain(regs, NULL, kernel, user, max_depth,
> - crosstask, false);
next prev parent reply other threads:[~2025-10-21 17:19 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-19 17:01 [RFC PATCH bpf-next v3 0/2] Pass external callchain entry to get_perf_callchain Tao Chen
2025-10-19 17:01 ` [PATCH bpf-next v3 1/2] perf: Use extern perf_callchain_entry for get_perf_callchain Tao Chen
2025-10-20 11:40 ` Peter Zijlstra
2025-10-22 15:14 ` Tao Chen
2025-10-19 17:01 ` [PATCH bpf-next v3 2/2] bpf: Use per-cpu BPF callchain entry to save callchain Tao Chen
2025-10-20 5:50 ` Tao Chen
2025-10-20 11:03 ` Peter Zijlstra [this message]
2025-10-22 15:59 ` Tao Chen
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