Re: [PATCH 2/2] io_uring: switch cancel_hash to use per list spinlock

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

On 5/30/22 07:38, Hao Xu wrote:
> On 5/30/22 06:50, Pavel Begunkov wrote:
>> On 5/29/22 19:40, Jens Axboe wrote:
>>> On 5/29/22 12:07 PM, Hao Xu wrote:
>>>> On 5/30/22 00:25, Jens Axboe wrote:
>>>>> On 5/29/22 10:20 AM, Hao Xu wrote:
>>>>>> From: Hao Xu <howeyxu@tencent.com>
>>>>>>
>>>>>> From: Hao Xu <howeyxu@tencent.com>
>>>>>>
>>>>>> Use per list lock for cancel_hash, this removes some completion lock
>>>>>> invocation and remove contension between different cancel_hash entries
>>>>>
>>>>> Interesting, do you have any numbers on this?
>>>>
>>>> Just Theoretically for now, I'll do some tests tomorrow. This is
>>>> actually RFC, forgot to change the subject.
>>>>
>>>>>
>>>>> Also, I'd make a hash bucket struct:
>>>>>
>>>>> struct io_hash_bucket {
>>>>>      spinlock_t lock;
>>>>>      struct hlist_head list;
>>>>> };
>>>>>
>>>>> rather than two separate structs, that'll have nicer memory locality too
>>>>> and should further improve it. Could be done as a prep patch with the
>>>>> old locking in place, making the end patch doing the per-bucket lock
>>>>> simpler as well.
>>>>
>>>> Sure, if the test number make sense, I'll send v2. I'll test the
>>>> hlist_bl list as well(the comment of it says it is much slower than
>>>> normal spin_lock, but we may not care the efficiency of poll
>>>> cancellation very much?).
>>>
>>> I don't think the bit spinlocks are going to be useful, we should
>>> stick with a spinlock for this. They are indeed slower and generally not
>>> used for that reason. For a use case where you need a ton of locks and
>>> saving the 4 bytes for a spinlock would make sense (or maybe not
>>> changing some struct?), maybe they have a purpose. But not for this.
>>
>> We can put the cancel hashes under uring_lock and completely kill
>> the hash spinlocking (2 lock/unlock pairs per single-shot). The code
>> below won't even compile and missing cancellation bits, I'll pick it
>> up in a week.
>>
>> Even better would be to have two hash tables, and auto-magically apply
>> the feature to SINGLE_SUBMITTER, SQPOLL (both will have uring_lock held)
>> and apoll (need uring_lock after anyway).
>>
>> @@ -7332,9 +7345,13 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
>>           return 0;
>>       }
>>
>> -    spin_lock(&ctx->completion_lock);
>> -    io_poll_req_insert(req);
>> -    spin_unlock(&ctx->completion_lock);
>> +    if (ctx->flags & IORING_MUTEX_HASH) {
> 
> Does IORING_MUTEX_HASH exclude IOSQE_ASYNC as well? though IOSQE_ASYNC
> is uncommon but users may do that.

It should, io-wq -> arm_poll() doesn't hold uring_lock, good point


-- 
Pavel Begunkov