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[82.132.237.201]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-57a35e86c36sm9511610a12.54.2024.06.05.12.20.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 05 Jun 2024 12:20:47 -0700 (PDT) Message-ID: <2027706f-971f-4552-aa0a-95c1db675cb2@gmail.com> Date: Wed, 5 Jun 2024 20:20:51 +0100 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHSET v2 0/7] Improve MSG_RING DEFER_TASKRUN performance To: Jens Axboe , io-uring@vger.kernel.org References: <20240530152822.535791-2-axboe@kernel.dk> <32ee0379-b8c7-4c34-8c3a-7901e5a78aa2@gmail.com> <656d487c-f0d8-401e-9154-4d01ef34356c@kernel.dk> <6c8ca196-2444-4c82-a8c0-a93f45fe47da@gmail.com> Content-Language: en-US From: Pavel Begunkov In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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