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

[Pavel Begunkov <asml.silence@gmail.com>]

On 6/5/24 17:41, Jens Axboe wrote:
> On 6/5/24 9:50 AM, Pavel Begunkov wrote:
>> On 6/4/24 19:57, Jens Axboe wrote:
>>> 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,
>>
>> Well, v3 tries to add msg_ring/nr_overflow handling to local
>> task work, that what I mean by duplicating paths, and we'll
>> continue gutting the hot path for supporting msg_ring in
>> this way.
> 
> No matter how you look at it, there will be changes to the hot path
> regardless of whether we use local task_work like in the original, or do
> the current approach.

The only downside for !msg_ring paths in the original was
un-inlining of local tw_add().

>> Does it work with eventfd? I can't find any handling, so next
>> you'd be adding:
>>
>> io_commit_cqring_flush(ctx);
> 
> That's merely because the flagging should be done in io_defer_wake(),
> moving that code to the common helper as well.

Flagging? If you mean io_commit_cqring_flush() then no,
it shouldn't and cannot be there. It's called strictly after
posting a CQE (or queuing an overflow), which is after tw
callback execution.

>> Likely draining around cq_extra should also be patched.
>> Yes, fixable, but it'll be a pile of fun, and without many
>> users, it'll take time to discover it all.
> 
> Yes that may need tweaking indeed. But this is a bit of a chicken and
> egg problem - there are not many users of it, because it currently
> sucks. We have to make it better, and there's already one user lined up
> because of these changes.
> 
> We can't just let MSG_RING linger. It's an appealing interface for
> message passing where you are using rings on both sides, but it's
> currently pretty much useless exactly for the case that we care about
> the most - DEFER_TASKRUN. So right now you are caught between a rock and
> a hard place, where you want to use DEFER_TASKRUN because it's a lot
> better for the things that people care about, but if you need message
> passing, then it doesn't work very well.
> 
>>> 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
>>
>> You can't get rid of target locking for fd passing, the file tables
>> are sync'ed by the lock. Otherwise it's only IOPOLL, because with
>> normal rings it can and IIRC does take the completion_lock for CQE
>> posting. I don't see a problem here, unless you care that much about
>> IOPOLL?
> 
> Right, fd passing still needs to grab the lock, and it still does with
> the patchset. We can't really get around it for fd passing, at least not
> without further work (of which I have no current plans to do). I don't
> care about IOPOLL in particular for message passing, I don't think there
> are any good use cases there. It's more of a code hygiene thing, the
> branches are still there and do exist.
> 
>>> 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
>>
>> Right, same as with CQEs
>>
>>> round trip, as that basically doubles the cost (and latency) of sending
>>
>> Sender's latency, which is IMHO not important at all
> 
> But it IS important. Not because of the latency itself, that part is
> less important, but because of the added overhead of bouncing from ring1
> to ring2, and then back from ring2 to ring1. The reduction in latency is
> a direct reflecting of the reduction of overhead.
> 
>>> 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.
>>
>> I'm not suggesting feeding that request into flush_completions()
>> and common completion infra, can be killed right in the tw callback.
> 
> Right, so you need to special case these requests when you run the local
> task_work. Which was my point above, you're going to need to accept hot
> path additions regardless of the approach.
> 

-- 
Pavel Begunkov