Re: [PATCH v3] ipc: Modify message queue accounting to not take kernel data structures into account

[Michael Kerrisk (man-pages)]

Hi David,

On 7 July 2015 at 07:16, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
> On Mon, 2015-07-06 at 17:49 +0200, Marcus Gelderie wrote:
>> A while back, the message queue implementation in the kernel was
>> improved to use btrees to speed up retrieval of messages (commit
>> d6629859b36). The patch introducing the improved kernel handling of
>> message queues (using btrees) has, as a by-product, changed the
>> meaning of the QSIZE field in the pseudo-file created for the queue.
>> Before, this field reflected the size of the user-data in the queue.
>> Since, it also takes kernel data structures into account. For
>> example, if 13 bytes of user data are in the queue, on my machine the
>> file reports a size of 61 bytes.
>>
>> There was some discussion on this topic before (for example
>> https://lkml.org/lkml/2014/10/1/115). Commenting on a th lkml, Michael
>> Kerrisk gave the following background (https://lkml.org/lkml/2015/6/16/74):
>>
>>     The pseudofiles in the mqueue filesystem (usually mounted at
>>     /dev/mqueue) expose fields with metadata describing a message
>>     queue. One of these fields, QSIZE, as originally implemented,
>>     showed the total number of bytes of user data in all messages in
>>     the message queue, and this feature was documented from the
>>     beginning in the mq_overview(7) page. In 3.5, some other (useful)
>>     work happened to break the user-space API in a couple of places,
>>     including the value exposed via QSIZE, which now includes a measure
>>     of kernel overhead bytes for the queue, a figure that renders QSIZE
>>     useless for its original purpose, since there's no way to deduce
>>     the number of overhead bytes consumed by the implementation.
>>     (The other user-space breakage was subsequently fixed.)
>
> Michael, this breakage was never finally documented in the manpage,
> right?

Right.

> I took a look and there is no mention, but it was a quick look.
> It's just that if this patch goes in, I'd hate ending up with something
> like this in the manpage:
>
> as of 3.5
>   <accounts for kernel overhead>
>
> as of 4.3
>   <behavior reverted back to not include kernel overhead... *sigh*>
>
> If there are changes to be made to the manpage, it should probably be
> posted with this patch, methinks.

I think something like the above will have to end up in the man page.
The only thing I'd add is that the fix also has gone to -stable
kernels. At least: I think this patch should also be marked for
-stable. I'll take care of the man page updates as the patch goes
through.

>> This patch removes the accounting of kernel data structures in the
>> queue. Reporting the size of these data-structures in the QSIZE field
>> was a breaking change (see Michael's comment above). Without the QSIZE
>> field reporting the total size of user-data in the queue, there is no
>> way to deduce this number.
>>
>> It should be noted that the resource limit RLIMIT_MSGQUEUE is counted
>> against the worst-case size of the queue (in both the old and the new
>> implementation). Therefore, the kernel overhead accounting in QSIZE is
>> not necessary to help the user understand the limitations RLIMIT imposes
>> on the processes.
>
> Also, I would suggest adding some comment in struct mqueue_inode_info
> for future reference, ie:
>
> -       unsigned long qsize; /* size of queue in memory (sum of all msgs) */
> +       /*
> +        * Size of queue in memory (sum of all msgs). Accounts for
> +        * only userspace overhead; ignoring any in-kernel rbtree nodes.
> +        */
> +       unsigned long qsize;
>
> But no big deal in any case.
>
> I think this is the right approach,

Me too.

> but would still like to know if Doug
> has any concerns about it.

Looking on gmane, Doug does not appear to have been active on any
lists since late May! Not sure why.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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