Message-ID: <b50ce7d2-b2a8-4552-8246-0464602bfd84@kernel.dk>
Date: Thu, 24 Oct 2024 08:40:37 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Pavel Begunkov <asml.silence@...il.com>, hexue <xue01.he@...sung.com>
Cc: io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8] io_uring: releasing CPU resources when polling

On 10/24/24 8:26 AM, Pavel Begunkov wrote:
> On 10/24/24 15:18, Jens Axboe wrote:
>> On 10/23/24 8:38 PM, hexue wrote:
>>> On 9/25/2024 12:12, Pavel Begunkov wrote:
> ...
>>> When the number of threads exceeds the number of CPU cores,the
>>> database throughput does not increase significantly. However,
>>> hybrid polling can releasing some CPU resources during the polling
>>> process, so that part of the CPU time can be used for frequent
>>> data processing and other operations, which speeds up the reading
>>> process, thereby improving throughput and optimizaing database
>>> performance.I tried different compression strategies and got
>>> results similar to the above table.(~30% throughput improvement)
>>>
>>> As more database applications adapt to the io_uring engine, I think
>>> the application of hybrid poll may have potential in some scenarios.
>>
>> Thanks for posting some numbers on that part, that's useful. I do
>> think the feature is useful as well, but I still have some issues
>> with the implementation. Below is an incremental patch on top of
>> yours to resolve some of those, potentially. Issues:
>>
>> 1) The patch still reads a bit like a hack, in that it doesn't seem to
>>     care about following the current style. This reads a bit lazy/sloppy
>>     or unfinished. I've fixed that up.
>>
>> 2) Appropriate member and function naming.
>>
>> 3) Same as above, it doesn't care about proper placement of additions to
>>     structs. Again this is a bit lazy and wasteful, attention should be
>>     paid to where additions are placed to not needlessly bloat
>>     structures, or place members in cache unfortunate locations. For
>>     example, available_time is just placed at the end of io_ring_ctx,
>>     why? It's a submission side member, and there's room with other
>>     related members. Not only is the placement now where you'd want it to
>>     be, memory wise, it also doesn't add 8 bytes to io_uring_ctx.
>>
>> 4) Like the above, the io_kiocb bloat is, by far, the worst. Seems to me
>>     that we can share space with the polling hash_node. This obviously
>>     needs careful vetting, haven't done that yet. IOPOLL setups should
>>     not be using poll at all. This needs extra checking. The poll_state
>>     can go with cancel_seq_set, as there's a hole there any. And just
>>     like that, rather than add 24b to io_kiocb, it doesn't take any extra
>>     space at all.
>>
>> 5) HY_POLL is a terrible name. It's associated with IOPOLL, and so let's
>>     please use a name related to that. And require IOPOLL being set with
>>     HYBRID_IOPOLL, as it's really a variant of that. Makes it clear that
>>     HYBRID_IOPOLL is really just a mode of operation for IOPOLL, and it
>>     can't exist without that.
>>
>> Please take a look at this incremental and test it, and then post a v9
>> that looks a lot more finished. Caveat - I haven't tested this one at
>> all. Thanks!
>>
>> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
>> index c79ee9fe86d4..6cf6a45835e5 100644
>> --- a/include/linux/io_uring_types.h
>> +++ b/include/linux/io_uring_types.h
>> @@ -238,6 +238,8 @@ struct io_ring_ctx {
>>           struct io_rings        *rings;
>>           struct percpu_ref    refs;
>>   +        u64            poll_available_time;
>> +
>>           clockid_t        clockid;
>>           enum tk_offsets        clock_offset;
>>   @@ -433,9 +435,6 @@ struct io_ring_ctx {
>>       struct page            **sqe_pages;
>>         struct page            **cq_wait_page;
>> -
>> -    /* for io_uring hybrid poll*/
>> -    u64            available_time;
>>   };
>>     struct io_tw_state {
>> @@ -647,9 +646,22 @@ struct io_kiocb {
>>         atomic_t            refs;
>>       bool                cancel_seq_set;
>> +    bool                poll_state;
> 
> As mentioned briefly before, that can be just a req->flags flag

That'd be even better, I generally despise random bool addition.

>>       struct io_task_work        io_task_work;
>> -    /* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
>> -    struct hlist_node        hash_node;
>> +    union {
>> +        /*
>> +         * for polled requests, i.e. IORING_OP_POLL_ADD and async armed
>> +         * poll
>> +         */
>> +        struct hlist_node    hash_node;
>> +        /*
>> +         * For IOPOLL setup queues, with hybrid polling
>> +         */
>> +        struct {
>> +            u64        iopoll_start;
>> +            u64        iopoll_end;
> 
> And IIRC it doesn't need to store the end as it's used immediately
> after it's set in the same function.

Nice, that opens up the door for less esoteric sharing as well. And
yeah, I'd just use:

runtime = ktime_get_ns() - req->iopoll_start - sleep_time;

in io_uring_hybrid_poll() and kill it entirely, doesn't even need a
local variable there. And then shove iopoll_start into the union with
comp_list/apoll_events.

My main points are really: don't randomly sprinkle additions to structs.
Think about if they are needed, and if they are, be a bit smarter about
where to place them. The original patch did neither of those, and that's
a non-starter.

-- 
Jens Axboe

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