From: Tao Chen <chen.dylane@linux.dev>
To: Peter Zijlstra <peterz@infradead.org>
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: Wed, 22 Oct 2025 23:59:32 +0800 [thread overview]
Message-ID: <cbee1f3b-49df-4228-898c-f6dc07e52add@linux.dev> (raw)
In-Reply-To: <20251020110303.GS3419281@noisy.programming.kicks-ass.net>
在 2025/10/20 19:03, Peter Zijlstra 写道:
> 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?
Yes it is, if we use per-cpu-bpf-callchain-entry like
bpf_bprintf_buffers, this is a proposal from Andrii and Alexei,
In my understanding, is it a low-probability event to be preempted three
times in a row in the same cpu?
>
>> - 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.
>
Alexei mentioned this rcu-lock issue before，
It seems we need preemption protection.
https://lore.kernel.org/bpf/CAADnVQ+s8B7-fvR1TNO-bniSyKv57cH_ihRszmZV7pQDyV=VDQ@mail.gmail.com
>> 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);
>
>
--
Best Regards
Tao Chen
prev parent reply other threads:[~2025-10-22 16:00 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
2025-10-22 15:59 ` Tao Chen [this message]
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