Re: [PATCH 3/3] 9p: Add mempools for RPCs

Christian Schoenebeck
2022-07-09T14:21:57Z

On Samstag, 9. Juli 2022 09:43:47 CEST Dominique Martinet wrote:
> I've taken the mempool patches to 9p-next
>
> Christian Schoenebeck wrote on Mon, Jul 04, 2022 at 03:56:55PM +0200:
> >> (I appreciate the need for testing, but this feels much less risky than
> >> the iovec series we've had recently... Famous last words?)
> >
> > Got it, consider my famous last words dropped. ;-)
>
> Ok, so I think you won this one...
>
> Well -- when testing normally it obviously works well, performance wise
> is roughly the same (obviously since it tries to allocate from slab
> first and in normal case that will work)
>
> When I tried gaming it with very low memory though I thought it worked
> well, but I managed to get a bunch of processes stuck in mempool_alloc
> with no obvious tid waiting for a reply.
> I had the bright idea of using fio with io_uring and interestingly the
> uring worker doesn't show up in ps or /proc/<pid>, but with qemu's gdb
> and lx-ps I could find a bunch of iou-wrk-<pid> that are all with
> similar stacks
> 1 │ [<0>] mempool_alloc+0x136/0x180
> 2 │ [<0>] p9_fcall_init+0x63/0x80 [9pnet]
> 3 │ [<0>] p9_client_prepare_req+0xa9/0x290 [9pnet]
> 4 │ [<0>] p9_client_rpc+0x64/0x610 [9pnet]
> 5 │ [<0>] p9_client_write+0xcb/0x210 [9pnet]
> 6 │ [<0>] v9fs_file_write_iter+0x4d/0xc0 [9p]
> 7 │ [<0>] io_write+0x129/0x2c0
> 8 │ [<0>] io_issue_sqe+0xa1/0x25b0
> 9 │ [<0>] io_wq_submit_work+0x90/0x190
> 10 │ [<0>] io_worker_handle_work+0x211/0x550
> 11 │ [<0>] io_wqe_worker+0x2c5/0x340
> 12 │ [<0>] ret_from_fork+0x1f/0x30
>
> or, and that's the interesting part
> 1 │ [<0>] mempool_alloc+0x136/0x180
> 2 │ [<0>] p9_fcall_init+0x63/0x80 [9pnet]
> 3 │ [<0>] p9_client_prepare_req+0xa9/0x290 [9pnet]
> 4 │ [<0>] p9_client_rpc+0x64/0x610 [9pnet]
> 5 │ [<0>] p9_client_flush+0x81/0xc0 [9pnet]
> 6 │ [<0>] p9_client_rpc+0x591/0x610 [9pnet]
> 7 │ [<0>] p9_client_write+0xcb/0x210 [9pnet]
> 8 │ [<0>] v9fs_file_write_iter+0x4d/0xc0 [9p]
> 9 │ [<0>] io_write+0x129/0x2c0
> 10 │ [<0>] io_issue_sqe+0xa1/0x25b0
> 11 │ [<0>] io_wq_submit_work+0x90/0x190
> 12 │ [<0>] io_worker_handle_work+0x211/0x550
> 13 │ [<0>] io_wqe_worker+0x2c5/0x340
> 14 │ [<0>] ret_from_fork+0x1f/0x30
>
> The problem is these flushes : the same task is holding a buffer for the
> original rpc and tries to get a new one, but waits for someone to free
> and.. obviously there isn't anyone (I cound 11 flushes pending, so more
> than the minimum number of buffers we'd expect from the mempool, and I
> don't think we missed any free)
>
> Now I'm not sure what's best here.
> The best thing to do would probably to just tell the client it can't use
> the mempools for flushes -- the flushes are rare and will use small
> buffers with your smaller allocations patch; I bet I wouldn't be able to
> reproduce that anymore but it should probably just forbid the mempool
> just in case.

So the problem is that one task ends up with more than 1 request at a time,
and the buffer is allocated and associated per request, not per task. If I am
not missing something, then this scenario (>1 request simultaniously per task)
currently may actually only happen with p9_client_flush() calls. Which
simplifies the problem.

So probably the best way would be to simply flip the call order such that
p9_tag_remove() is called before p9_client_flush(), similar to how it's
already done with p9_client_clunk() calls?

> Anyway, I'm not comfortable with this patch right now, a hang is worse
> than an allocation failure warning.

As you already mentioned, with the pending 'net/9p: allocate appropriate
reduced message buffers' patch those hangs should not happen, as Tflush would
then just kmalloc() a small buffer. But I would probably still fix this issue
here nevertheless, as it might hurt in other ways in future. Shouldn't be too
much noise to swap the call order, right?

> > > > How about I address the already discussed issues and post a v5 of
> > > > those
> > > > patches this week and then we can continue from there?
> > >
> > > I would have been happy to rebase your patches 9..12 on top of Kent's
> > > this weekend but if you want to refresh them this week we can continue
> > > from there, sure.
> >
> > I'll rebase them on master and address what we discussed so far. Then
> > we'll
> > see.
>
> FWIW and regarding the other thread with virito queue sizes, I was only
> considering the later patches with small RPCs for this merge window.

I would also recommend to leave out the virtio patches, yes.

> Shall we try to focus on that first, and then revisit the virtio and
> mempool patches once that's done?

Your call. I think both ways are viable.

Best regards,
Christian Schoenebeck