linux-kernel - Re: [PATCH v2 RESEND] io_uring/fdinfo: add timeout

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Message-ID: <20241012091026.1824-1-ruyi.zhang@samsung.com>
Date: Sat, 12 Oct 2024 09:10:25 +0000
From: Ruyi Zhang <ruyi.zhang@...sung.com>
To: asml.silence@...il.com
Cc: axboe@...nel.dk, io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
	peiwei.li@...sung.com, ruyi.zhang@...sung.com
Subject: Re: [PATCH v2 RESEND] io_uring/fdinfo: add timeout_list to fdinfo

---
On 2024-10-10 15:35 Pavel Begunkov wrote:
>> Two questions:
>> 
>> 1. I agree with you, we shouldn't walk a potentially very
>> long list under spinlock. but i can't find any other way
>> to get all the timeout

> If only it's just under the spin, but with disabled irqs...

>> information than to walk the timeout_list. Do you have any
>> good ideas?

> In the long run it'd be great to replace the spinlock
> with a mutex, i.e. just ->uring_lock, but that would might be
> a bit involving as need to move handling to the task context.

 Yes, it makes more sense to replace spin_lock, but that would
 require other related logic to be modified, and I don't think
 it's wise to do that for the sake of a piece of debugging
 information.

>> 2. I also agree seq_printf heavier, if we use
>> seq_put_decimal_ull and seq_puts to concatenate strings,
>> I haven't tested whether it's more efficient or not, but
>> the code is certainly not as readable as the former. It's
>> also possible that I don't fully understand what you mean
>> and want to hear your opinion.

> I don't think there is any difference, it'd be a matter of
> doubling the number of in flight timeouts to achieve same
> timings. Tell me, do you really have a good case where you
> need that (pretty verbose)? Why not drgn / bpftrace it out
> of the kernel instead?

 Of course, this information is available through existing tools.
 But I think that most of the io_uring metadata has been exported
 from the fdinfo file, and the purpose of adding the timeout
 information is the same as before, easier to use. This way, 
 I don't have to write additional scripts to get all kinds of data.

 And as far as I know, the io_uring_show_fdinfo function is
 only called once when the user is viewing the 
 /proc/xxx/fdinfo/x file once. I don't think we normally need to 
 look at this file as often, and only look at it when the program
 is abnormal, and the timeout_list is very long in the extreme case,
 so I think the performance impact of adding this code is limited.

---
Ruyi Zhang