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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jens Axboe <axboe@kernel.dk>, Stefan Metzmacher <metze@samba.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>, linux-kernel@vger.kernel.org,
        io-uring@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH v2] io_thread/x86: setup io_threads more like normal user space threads
In-Reply-To: <df4b116a-3324-87b7-ff40-67d134b4e55c@kernel.dk>
References: <20210411152705.2448053-1-metze@samba.org> <20210505110310.237537-1-metze@samba.org> <df4b116a-3324-87b7-ff40-67d134b4e55c@kernel.dk>
Date:   Wed, 05 May 2021 23:57:18 +0200
Message-ID: <878s4soncx.ffs@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, May 05 2021 at 15:24, Jens Axboe wrote:
> On 5/5/21 5:03 AM, Stefan Metzmacher wrote:
>> As io_threads are fully set up USER threads it's clearer to
>> separate the code path from the KTHREAD logic.
>> 
>> The only remaining difference to user space threads is that
>> io_threads never return to user space again.
>> Instead they loop within the given worker function.
>> 
>> The fact that they never return to user space means they
>> don't have an user space thread stack. In order to
>> indicate that to tools like gdb we reset the stack and instruction
>> pointers to 0.
>> 
>> This allows gdb attach to user space processes using io-uring,
>> which like means that they have io_threads, without printing worrying
>> message like this:
>> 
>>   warning: Selected architecture i386:x86-64 is not compatible with reported target architecture i386
>> 
>>   warning: Architecture rejected target-supplied description
>> 
>> The output will be something like this:
>> 
>>   (gdb) info threads
>>     Id   Target Id                  Frame
>>   * 1    LWP 4863 "io_uring-cp-for" syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
>>     2    LWP 4864 "iou-mgr-4863"    0x0000000000000000 in ?? ()
>>     3    LWP 4865 "iou-wrk-4863"    0x0000000000000000 in ?? ()
>>   (gdb) thread 3
>>   [Switching to thread 3 (LWP 4865)]
>>   #0  0x0000000000000000 in ?? ()
>>   (gdb) bt
>>   #0  0x0000000000000000 in ?? ()
>>   Backtrace stopped: Cannot access memory at address 0x0
>
> I have queued this one up in the io_uring branch, also happy to drop it if
> the x86 folks want to take it instead. Let me know!

I have no objections, but heck what's the rush here?

Waiting a day for the x86 people to respond it not too much asked for
right?

Thanks,

        tglx