linux-kernel - Re: [PATCH v12 06/14] unwind_user/deferred: Add deferred unwinding interface

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Message-ID: <16b8f9a8-b1f8-43ac-9dad-4b83d8ca9f9f@efficios.com>
Date: Wed, 2 Jul 2025 15:48:16 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org, x86@...nel.org,
 Masami Hiramatsu <mhiramat@...nel.org>, Josh Poimboeuf
 <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>,
 Indu Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>,
 Beau Belgrave <beaub@...ux.microsoft.com>, Jens Remus
 <jremus@...ux.ibm.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH v12 06/14] unwind_user/deferred: Add deferred unwinding
 interface

On 2025-07-02 15:40, Steven Rostedt wrote:
> On Wed, 2 Jul 2025 15:36:00 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>> union unwind_task_id {
>> 	struct {
>> 		u32		task_id;
>> 		u32		cnt;
>> 	}
>> 	u64 id;
>> };
>>
>> static u64 get_cookie(struct unwind_task_info *info)
>> {
>> 	u32 cnt = READ_ONCE(info->id.cnt);
>> 	u32 new_cnt;
>>
>> 	if (cnt & 1)
>> 		return info->id;
>>
>> 	if (unlikely(!info->id.task_id)) {
>> 		u32 task_id = local_clock();
>>
>> 		cnt = 0;
>> 		if (try_cmpxchg(&info->id.task_id, &cnt, task_id))
>> 			task_id = cnt;
>> 	}
>>
>> 	new_cnt = cnt + 3;
>> 	if (try_cmpxchg(&info->id, &cnt, new_cnt))
>> 		new_cnt = cnt; // try_cmpxchg() expects something
>>
>> 	return info->id;
>> }
> 
> Honestly I think this is way overkill. What I would do, is to have the
> cookie saved in the event be 64 bit, but we can start with the
> simple 32 bit solution keeping the top 32 bits zeros. If this does
> indeed become an issue in the future, we could fix it with a 64 bit
> number. By making sure all the exposed "cookies" are 64 bit, it should
> not break anything. The cookie is just supposed to be a random unique
> number that associates a request with its deferred user space stack
> trace.
> 
> With any exposed cookies to user space being 64 bits, this should not
> be an issue to address in the future.

FWIW, I liked your idea of making the cookie 64-bit with:

- 32-bit cpu number,
- 32-bit per-CPU free running counter.

This is simple, works on 32-bit archs, and it does not overflow as often
as time LSB because it counts execution contexts.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com