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

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

On 5/30/22 00:34, Jens Axboe wrote:
> 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?

It replaces 2 spin lock/unlock with one io_tw_lock() in the completion
path, which is done once per tw batch and grabbed anyway if
there is no contention (see handle_tw_list()).

It could be unconditional, but I'd say for those 3 cases we have
non-existing chance to regress perf/latency, but I can think of
some cases where it might screw latencies, all share io_uring
b/w threads.

Should benefit the cancellation path as well, but I don't care
about it as well.

> What about io_poll_remove_all()?

As mentioned, it's not handled in the diff, but easily doable,
it should just traverse both hash tables.

-- 
Pavel Begunkov