Re: io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx

Thomas Gleixner
2020-05-21T18:39:37Z

Ming,

Ming Lei <ming.lei@redhat.com> writes:
> On Thu, May 21, 2020 at 10:13:59AM +0200, Thomas Gleixner wrote:
>> Ming Lei <ming.lei@redhat.com> writes:
>> > On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote:
>> > - otherwise, the kthread just retries and retries to allocate & release,
>> > and sooner or later, its time slice is consumed, and migrated out, and the
>> > cpu hotplug handler will get chance to run and move on, then the cpu is
>> > shutdown.
>>
>> 1) This is based on the assumption that the kthread is in the SCHED_OTHER
>> scheduling class. Is that really a valid assumption?
>
> Given it is unlikely path, we can add msleep() before retrying when INACTIVE bit
> is observed by current thread, and this way can avoid spinning and should work
> for other schedulers.

That should work, but pretty is something else

>>
>> 2) What happens in the following scenario:
>>
>> unplug
>>
>> mq_offline
>> set_ctx_inactive()
>> drain_io()
>>
>> io_kthread()
>> try_queue()
>> wait_on_ctx()
>>
>> Can this happen and if so what will wake up that thread?
>
> drain_io() releases all tag of this hctx, then wait_on_ctx() will be waken up
> after any tag is released.

drain_io() is already done ...

So looking at that thread function:

static int io_sq_thread(void *data)
{
struct io_ring_ctx *ctx = data;

while (...) {
....
to_submit = io_sqring_entries(ctx);

--> preemption

hotplug runs
mq_offline()
set_ctx_inactive();
drain_io();
finished();

--> thread runs again

mutex_lock(&ctx->uring_lock);
ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
mutex_unlock(&ctx->uring_lock);

....

if (!to_submit || ret == -EBUSY)
...
wait_on_ctx();

Can this happen or did drain_io() already take care of the 'to_submit'
items and the call to io_submit_sqes() turns into a zero action ?

If the above happens then nothing will wake it up because the context
draining is done and finished.

Thanks,

tglx