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/ -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
