Re: [PATCHSET v2 0/7] Improve MSG_RING DEFER_TASKRUN performance

[Jens Axboe <axboe@kernel.dk>]

On 6/3/24 7:53 AM, Pavel Begunkov wrote:
> On 5/30/24 16:23, Jens Axboe wrote:
>> Hi,
>>
>> For v1 and replies to that and tons of perf measurements, go here:
> 
> I'd really prefer the task_work version rather than carving
> yet another path specific to msg_ring. Perf might sounds better,
> but it's duplicating wake up paths, not integrated with batch
> waiting, not clear how affects different workloads with target
> locking and would work weird in terms of ordering.

The duplication is really minor, basically non-existent imho. It's a
wakeup call, it's literally 2 lines of code. I do agree on the batching,
though I don't think that's really a big concern as most usage I'd
expect from this would be sending single messages. You're not batch
waiting on those. But there could obviously be cases where you have a
lot of mixed traffic, and for those it would make sense to have the
batch wakeups.

What I do like with this version is that we end up with just one method
for delivering the CQE, rather than needing to split it into two. And it
gets rid of the uring_lock double locking for non-SINGLE_ISSUER. I know
we always try and push people towards DEFER_TASKRUN|SINGLE_ISSUER, but
that doesn't mean we should just ignore the cases where that isn't true.
Unifying that code and making it faster all around is a worthy goal in
and of itself. The code is CERTAINLY a lot cleaner after the change than
all the IOPOLL etc.

> If the swing back is that expensive, another option is to
> allocate a new request and let the target ring to deallocate
> it once the message is delivered (similar to that overflow
> entry).

I can give it a shot, and then run some testing. If we get close enough
with the latencies and performance, then I'd certainly be more amenable
to going either route.

We'd definitely need to pass in the required memory and avoid the return
round trip, as that basically doubles the cost (and latency) of sending
a message. The downside of what you suggest here is that while that
should integrate nicely with existing local task_work, it'll also mean
that we'll need hot path checks for treating that request type as a
special thing. Things like req->ctx being not local, freeing the request
rather than recycling, etc. And that'll need to happen in multiple
spots.

-- 
Jens Axboe