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

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

> > > 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.

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


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