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

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Message-ID: <bb29a10c-6130-4040-8521-0b5375c017ef@kylinos.cn>
Date: Fri, 30 Jan 2026 09:35:26 +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 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.

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