linux-kernel - Re: [PATCH v4 19/39] unwind_user/sframe: Add support for reading .sframe contents

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Message-ID: <bcb5ddee-c8d3-4f2b-9d84-8a2e1f1236fa@linux.ibm.com>
Date: Tue, 28 Jan 2025 11:54:11 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
        Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Florian Weimer <fweimer@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 19/39] unwind_user/sframe: Add support for reading
 .sframe contents

On 28.01.2025 01:39, Andrii Nakryiko wrote:
> On Fri, Jan 24, 2025 at 1:41 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>> On Fri, Jan 24, 2025 at 10:02:46AM -0800, Andrii Nakryiko wrote:
>>> On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:

>>>> +       fre_addr = sec->fres_start + fde->fres_off;
>>>> +
>>>> +       for (i = 0; i < fde->fres_num; i++) {
>>>
>>> why not binary search? seem more logical to guard against cases with
>>> lots of FREs and be pretty fast in common case anyways.
>>
>> That would be nice, but the FREs are variable-sized and you don't know
>> how big one is until you start reading it.
> 
> ah, another non-obvious thing, yeah... do you think it's worth fixing
> this and making FREs binary searchable in v3?
> 
> Indu, do you have some stats on distribution of FRE count per FDE in practice?
> 
> Tbh, FRE format is bothering me quite a lot... but let's discuss that
> in another thread with you and Indu

I would be interested to be part of that discussion.  I could share some
preliminary stats from my s390 work.

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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