Re: [PATCH 2/2] io_uring: support multiple rings to share same poll thread by specifying same cpu

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi Jens,

> On 10/20/20 2:23 AM, Xiaoguang Wang wrote:
>> We have already supported multiple rings to share one same poll thread
>> by passing IORING_SETUP_ATTACH_WQ, but it's not that convenient to use.
>> IORING_SETUP_ATTACH_WQ needs users to ensure that a parent ring instance
>> has already existed, that means it will require app to regulate the
>> creation oder between uring instances.
>>
>> Currently we can make this a bit simpler, for those rings which will
>> have SQPOLL enabled and are willing to be bound to one same cpu, add a
>> capability that these rings can share one poll thread by specifying
>> a new IORING_SETUP_SQPOLL_PERCPU flag, then we have 3 cases
>>    1, IORING_SETUP_ATTACH_WQ: if user specifies this flag, we'll always
>> try to attach this ring to an existing ring's corresponding poll thread,
>> no matter whether IORING_SETUP_SQ_AFF or IORING_SETUP_SQPOLL_PERCPU is
>> set.
>>    2, IORING_SETUP_SQ_AFF and IORING_SETUP_SQPOLL_PERCPU are both enabled,
>> for this case, we'll create a single poll thread to be shared by these
>> rings, and this poll thread is bound to a fixed cpu.
>>    3, for any other cases, we'll just create one new poll thread for the
>> corresponding ring.
>>
>> And for case 2, don't need to regulate creation oder of multiple uring
>> instances, we use a mutex to synchronize creation, for example, say five
>> rings which all have IORING_SETUP_SQ_AFF & IORING_SETUP_SQPOLL_PERCPU
>> enabled, and are willing to be bound same cpu, one ring that gets the
>> mutex lock will create one poll thread, the other four rings will just
>> attach themselves the previous created poll thread once they get lock
>> successfully.
>>
>> To implement above function, define a percpu io_sq_data array:
>>      static struct io_sq_data __percpu *percpu_sqd;
>> When IORING_SETUP_SQ_AFF and IORING_SETUP_SQPOLL_PERCPU are both enabled,
>> we will use struct io_uring_params' sq_thread_cpu to locate corresponding
>> sqd, and use this sqd to save poll thread info.
> 
> Do you have any test results?
First sorry for late reply.
Yes, and I'll attach it in next version patch.

> 
> Not quite clear to me, but if IORING_SETUP_SQPOLL_PERCPU is set, I think
> it should always imply IORING_SETUP_ATTACH_WQ in the sense that it would
> not make sense to have more than one poller thread that's bound to a
> single CPU, for example.
Yes, agree, in this case, it's meaningful that IORING_SETUP_SQPOLL_PERCPU
implies IORING_SETUP_ATTACH_WQ, but that needs app already has a sqpoll
io_uring instance bound to same cpu. I think it doesn't matter much, app
can pass IORING_SETUP_SQPOLL_PERCPU and IORING_SETUP_ATTACH_WQ, then
we will try to attach an exsiting sqpoll io_uring instance, and ignore
IORING_SETUP_SQPOLL_PERCPU.

The key benefit for IORING_SETUP_SQPOLL_PERCPU is that it does not require
app to regulate the creation oder between uring instances, IORING_SETUP_ATTACH_WQ
needs app creats a sqpoll io_uring instance, follwing io_uring instances attach
to it, it's not that convenient, needs to pass fd between threads in a process.
If we support to share one poll thread between processes, it's also not convenient.

And I have another question about the feature that make multiple ctxs share
one poll thread, do we need that these ctxs belong to one single process, or
can ctxs belong to different process share one poll thread?
   Seems that current mainline codes allow it, say a parent process creates a
   sqpoll io_uring instance, and this parent process fork a child process. This
   child process will inherit parent process' files, it may create a new sqpoll
   io_uring instance and attach it to the previous created io_uring instance,
   then parent and child share one poll thread.

Also in current mainline codes:
list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
	if (current->cred != ctx->creds) {
		if (old_cred)
			revert_creds(old_cred);
		old_cred = override_creds(ctx->creds);
	}
	io_sq_thread_associate_blkcg(ctx, &cur_css);
#ifdef CONFIG_AUDIT
	current->loginuid = ctx->loginuid;
	current->sessionid = ctx->sessionid;
#endif

	ret |= __io_sq_thread(ctx, start_jiffies, cap_entries);

	io_sq_thread_drop_mm_files();
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Here you always drop mm, files and nsproxy once finishing every ctx's process, is it
because you design that ctxs belong to different process share one poll thread, so you
switch every ctx's mm, files and nsproxy when handling it?

}

Regards,
Xiaoguang Wang


> 
>> @@ -6814,8 +6819,17 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
>>   	return 0;
>>   }
>>   
>> -static void io_put_sq_data(struct io_sq_data *sqd)
>> +static void io_put_sq_data(struct io_ring_ctx *ctx, struct io_sq_data *sqd)
>>   {
>> +	int percpu_sqd = 0;
>> +
>> +	if ((ctx->flags & IORING_SETUP_SQ_AFF) &&
>> +	    (ctx->flags & IORING_SETUP_SQPOLL_PERCPU))
>> +		percpu_sqd = 1;
>> +
>> +	if (percpu_sqd)
>> +		mutex_lock(&sqd->percpu_sq_lock);
>> +
>>   	if (refcount_dec_and_test(&sqd->refs)) {
>>   		/*
>>   		 * The park is a bit of a work-around, without it we get
> 
> For this, and the setup, you should make it dynamic. Hence don't
> allocate the percpu data etc until someone asks for it, and when the
> last user of it goes away, it should go away as well.
> 
> That would make the handling of it identical to what we currently have,
> and no need to special case any of this like you do above.
> 
>