linux-kernel - Re: [PATCH v4 5/5] blktrace: Make init_blk_tracer() asynchronous when trace_async

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Message-ID: <a61e6bc0-58d1-46f0-8a4f-17f02503fe17@kylinos.cn>
Date: Fri, 30 Jan 2026 11:09:26 +0800
From: Yaxiong Tian <tianyaxiong@...inos.cn>
To: Steven Rostedt <rostedt@...dmis.org>, Jens Axboe <axboe@...nel.dk>
Cc: mhiramat@...nel.org, mathieu.desnoyers@...icios.com, corbet@....net,
 skhan@...uxfoundation.org, linux-trace-kernel@...r.kernel.org,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 5/5] blktrace: Make init_blk_tracer() asynchronous when
 trace_async_init set


在 2026/1/30 09:35, Yaxiong Tian 写道:
>
> 在 2026/1/30 04:29, Steven Rostedt 写道:
>> On Wed, 28 Jan 2026 19:25:46 -0700
>> Jens Axboe <axboe@...nel.dk> wrote:
>>
>>> On Jan 28, 2026, at 5:40 PM, Steven Rostedt <rostedt@...dmis.org> 
>>> wrote:
>>>> 
>>>> Jens,
>>>>
>>>> Can you give me an acked-by on this patch and I can take the series 
>>>> through
>>>> my tree.
>>> On phone, hope this works:
>>>
>>> Acked-by: Jens Axboe <axboe@...nel.dk>
>> Thanks!
>>
>>>> Or perhaps this doesn't even need to test the trace_async_init flag 
>>>> and can
>>>> always do the work queue? Does blk_trace ever do tracing at boot 
>>>> up? That
>>>> is, before user space starts?
>>> Not via the traditonal way of running blktrace.
>> Masami and Yaxiong,
>>
>> I've been thinking about this more and I'm not sure we need the
>> trace_async_init kernel parameter at all. As blktrace should only be
>> enabled by user space, it can always use the work queue.
>>
>> For kprobes, if someone is adding a kprobe on the kernel command 
>> line, then
>> they are already specifying that tracing is more important.
>>
>> Patch 3 already keeps kprobes from being an issue with contention of the
>> tracing locks, so I don't think it ever needs to use the work queue.
>>
>> Wouldn't it just be better to remove the trace_async_init and make 
>> blktrace
>> always use the work queue and kprobes never do it (but exit out early if
>> there were no kprobes registered)?
>>
>> That is, remove patch 2 and 4 and make this patch always use the work 
>> queue.
>
> Yesterday, I was curious about|trace_event_update_all()|, so I 
> added|pr_err(xx)|prints within the function's loop. I discovered that 
> these prints appeared as late as 14 seconds later (printing is 
> time-consuming), by which time the desktop had already been up for 
> quite a while. However,|trace_eval_sync()|had already finished running 
> at 0.6 seconds.
>
> This implies that I originally 
> thought|trace_eval_sync()|'s|destroy_workqueue()|would wait for all 
> tasks to complete, but it seems that might not be the case. From this, 
> if the above conclusion is true, then strictly speaking, tasks 
> using|queue_work(xx)|cannot be guaranteed to finish before the init 
> process executes. If it's necessary to strictly ensure initialization 
> completes before user space starts, 
> using|async_synchronize_full()|or|async_synchronize_full_domain()|would 
> be better in such scenarios.
I need to double-check this issue—theoretically, it shouldn't exist. But 
I'm not sure why the print appeared at the 14-second mark.
>
> Of course, the situation described above is an extreme case. I don't 
> oppose this approach; I only hope to make the startup faster for 
> ordinary users who don’t use trace, while minimizing the impact on 
> others as much as possible.
>
>>
>> -- Steve