linux-kernel - Re: [PATCH bpf-next v4 2/2] bpf: Use per-cpu BPF callchain entry to save callchain

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Message-ID: <e2e9b239-784f-411f-bd1b-8fcf103609ff@linux.dev>
Date: Thu, 23 Oct 2025 00:08:32 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Kan Liang <kan.liang@...ux.intel.com>, Song Liu <song@...nel.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 "linux-perf-use." <linux-perf-users@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v4 2/2] bpf: Use per-cpu BPF callchain entry to
 save callchain

在 2025/10/22 01:28, Alexei Starovoitov 写道:
> On Tue, Oct 21, 2025 at 9:07 AM Tao Chen <chen.dylane@...ux.dev> 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.
>>
>> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
>> ---
>>   kernel/bpf/stackmap.c | 98 ++++++++++++++++++++++++++++++++-----------
>>   1 file changed, 73 insertions(+), 25 deletions(-)
>>
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 94e46b7f340..97028d39df1 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -31,6 +31,52 @@ struct bpf_stack_map {
>>          struct stack_map_bucket *buckets[] __counted_by(n_buckets);
>>   };
>>
>> +struct bpf_perf_callchain_entry {
>> +       u64 nr;
>> +       u64 ip[PERF_MAX_STACK_DEPTH];
>> +};
>> +
>> +#define MAX_PERF_CALLCHAIN_PREEMPT 3
>> +static DEFINE_PER_CPU(struct bpf_perf_callchain_entry[MAX_PERF_CALLCHAIN_PREEMPT],
>> +                     bpf_perf_callchain_entries);
>> +static DEFINE_PER_CPU(int, bpf_perf_callchain_preempt_cnt);
> 
> This is too much extra memory. Above adds 1k * 3 * num_cpus.
> Let's reuse perf callchains.
> Especially since they're controlled by perf_event_max_stack sysctl.
> See Peter's suggestion in v3.
> And for the future don't respin so quickly.

Ok, let's base our discussion on v3, sorry for the overly rapid version 
iterations impacted the maintainers' review process, i will respin 
slowly, thanks.

-- 
Best Regards
Tao Chen