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

[Jens Axboe <axboe@kernel.dk>]

On 5/29/22 4:50 PM, 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).

My hope was that it'd take us closer to being able to use more granular
locking for hashing in general. I don't care too much about the
cancelation, but the normal hash locking would be useful to do.

However, for cancelations, under uring_lock would indeed be preferable
to doing per-bucket locks there. Guess I'll wait and see what your final
patch looks like, not sure why it'd be a ctx conditional?

What about io_poll_remove_all()?

-- 
Jens Axboe