Re: Occasional delayed output of events

On Mon, Jan 18, 2021 at 9:31 AM Steve Grubb <sgrubb(a)redhat.com&gt; wrote: > On Monday, January 18, 2021 8:54:30 AM EST Paul Moore wrote: > > > > > I like the N of M concept but there would be a LOT of change > > > > > -especiallyfor all the non-kernel event sources. The EOE would be the > > > > > mostseamless, but at a cost. My preference is to allow the 2 second > > > > > 'timer'to be configurable. > > > > > > > > Agree with Burn, numbering the records coming up from the kernel isgoing > > > > to be a real nightmare, and not something to consider lightly.Especially > > > > when it sounds like we don't yet have a root cause for theissue. > > > > > > A very long time ago, we had numbered records. But it was decided > > > thatthere's no real point in it and we'd rather just save disk space. > > > > With the current kernel code, adding numbered records is not something totake > > lightly. > > That's why I'm saying we had it and it was removed. I could imagine that ifyou > had auditing of the kill syscall enabled and a whole process group wasbeing > killed, you could have hundreds of records that need numbering. No goodway to > know in advance how many records make up the event. You only mentioned disk space concerns so it wasn't clear to me thatyou were in agreement about this being a bad idea. Regardless, I'mglad to see we are on the same page about this. > > > I know that the kernel does not serialize the events headed for userspace. > > > But I'm curious how an event gets stuck and others can jump aheadwhile one > > > that's already inflight can get hung for 4 seconds before it'snext record > > > goes out? > > > > Have you determined that the problem is the kernel? > > I assume so because the kernel adds the timestamp and choses what hits thesocket > next. Auditd does no ordering of events. It just looks up the textevent ID, some > minor translation if the enriched format is being used, andwrites it to disk. It > can handle well over 100k records per second. Feel free to insert the old joke about assumptions. I guess I was hoping for a bit more understanding of the problem andperhaps some actual data indicating the kernel was the source of theproblem. Conjecture based on how things are supposed to work can bemisleading. > > Initially it was looking like it was a userspace issue, is that no longerthe > > general thought? > > I don't see how user space could cause this. Even if auditd was slow, > itshouldn't take 4 seconds to write to disk and then come back to read > anotherrecord. And even it did, why would the newest record go out before > completingone that's in progress? Something in the kernel chooses what's next. > Isuspect that might need looking at. See above. > > Also, is there a reliable reproducer yet? > > I don't know of one. But, I suppose we could modify ausearch to look forexamples > of this. The kernel queuing is a rather complicated affair due to the need togracefully handle auditd failing, fallbacks to the console, andmulticast groups all while handling extreme pressure (e.g. auditing*every* syscall) and not destroying the responsiveness of the system(we actually can still make forward progress if you are auditing*every* syscall). With that complexity comes a number of cornercases, and I imagine there are a few cases where the system is underextreme pressure and/or the auditd daemon is dead and/or starved fromCPU time. As I know Richard is reading this, to be clear I'm talkingabout the hold/retry queues and the UNICAST_RETRIES case. The severedelays you are talking about in this thread seem severe, but perhapsif the system is under enough pressure to cause the ordering issues inthe first place such a delay is to be expected. Anyway, my test setup isn't likely able to reproduce such a scenariowithout some significant tweaks, so perhaps those of you who have seenthis problem (Burn, and anyone else?) could shed some light into thestate of the system when the ordering problem occurred.