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

[Hao Xu <haoxu.linux@icloud.com>]

On 5/30/22 14:38, 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).
>>
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 6be21967959d..191fa7f31610 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -7120,12 +7120,20 @@ static void io_poll_task_func(struct io_kiocb 
>> *req, bool *locked)
>>       }
>>
>>       io_poll_remove_entries(req);
>> -    spin_lock(&ctx->completion_lock);
>> -    hash_del(&req->hash_node);
>> -    __io_req_complete_post(req, req->cqe.res, 0);
>> -    io_commit_cqring(ctx);
>> -    spin_unlock(&ctx->completion_lock);
>> -    io_cqring_ev_posted(ctx);
>> +
>> +    if (ctx->flags & IORING_MUTEX_HASH) {
>> +        io_tw_lock(ctx, locked);
>> +        hash_del(&req->hash_node);
>> +        io_req_complete_state(req, req->cqe.res, 0);
>> +        io_req_add_compl_list(req);
>> +    } else {
>> +        spin_lock(&ctx->completion_lock);
>> +        hash_del(&req->hash_node);
>> +        __io_req_complete_post(req, req->cqe.res, 0);
>> +        io_commit_cqring(ctx);
>> +        spin_unlock(&ctx->completion_lock);
>> +        io_cqring_ev_posted(ctx);
>> +    }
>>   }
>>
>>   static void io_apoll_task_func(struct io_kiocb *req, bool *locked)
>> @@ -7138,9 +7146,14 @@ static void io_apoll_task_func(struct io_kiocb 
>> *req, bool *locked)
>>           return;
>>
>>       io_poll_remove_entries(req);
>> -    spin_lock(&ctx->completion_lock);
>> -    hash_del(&req->hash_node);
>> -    spin_unlock(&ctx->completion_lock);
>> +    if (ctx->flags & IORING_MUTEX_HASH) {
>> +        io_tw_lock(ctx, locked);
>> +        hash_del(&req->hash_node);
>> +    } else {
>> +        spin_lock(&ctx->completion_lock);
>> +        hash_del(&req->hash_node);
>> +        spin_unlock(&ctx->completion_lock);
>> +    }
>>
>>       if (!ret)
>>           io_req_task_submit(req, locked);
>> @@ -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.

I see, cases like IOSQE_ASYNC definitely goes into the else branch since
it's in iowq context.

> 
>> +        io_poll_req_insert(req);
>> +    } else {
>> +        spin_lock(&ctx->completion_lock);
>> +        io_poll_req_insert(req);
>> +        spin_unlock(&ctx->completion_lock);
>> +    }
>>
>>       if (mask) {
>>           /* can't multishot if failed, just queue the event we've got */
>