linux-kernel - Re: [PATCH 0/2] Don't show PF_IO

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Date:   Thu, 25 Mar 2021 13:40:34 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     io-uring@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, oleg@...hat.com, metze@...ba.org
Subject: Re: [PATCH 0/2] Don't show PF_IO_WORKER in /proc/<pid>/task/

On 3/25/21 1:33 PM, Eric W. Biederman wrote:
> Jens Axboe <axboe@...nel.dk> writes:
> 
>> Hi,
>>
>> Stefan reports that attaching to a task with io_uring will leave gdb
>> very confused and just repeatedly attempting to attach to the IO threads,
>> even though it receives an -EPERM every time. This patchset proposes to
>> skip PF_IO_WORKER threads as same_thread_group(), except for accounting
>> purposes which we still desire.
>>
>> We also skip listing the IO threads in /proc/<pid>/task/ so that gdb
>> doesn't think it should stop and attach to them. This makes us consistent
>> with earlier kernels, where these async threads were not related to the
>> ring owning task, and hence gdb (and others) ignored them anyway.
>>
>> Seems to me that this is the right approach, but open to comments on if
>> others agree with this. Oleg, I did see your messages as well on SIGSTOP,
>> and as was discussed with Eric as well, this is something we most
>> certainly can revisit. I do think that the visibility of these threads
>> is a separate issue. Even with SIGSTOP implemented (which I did try as
>> well), we're never going to allow ptrace attach and hence gdb would still
>> be broken. Hence I'd rather treat them as separate issues to attack.
> 
> A quick skim shows that these threads are not showing up anywhere in
> proc which appears to be a problem, as it hides them from top.
> 
> Sysadmins need the ability to dig into a system and find out where all
> their cpu usage or io's have gone when there is a problem.  I general I
> think this argues that these threads should show up as threads of the
> process so I am not even certain this is the right fix to deal with gdb.

That's a good point, overall hiding was not really what I desired, just
getting them out of gdb's hands. And arguably it _is_ a gdb bug, but...

-- 
Jens Axboe