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

Ming Lei
2020-05-22T01:57:40Z

On Thu, May 21, 2020 at 08:39:16PM +0200, Thomas Gleixner wrote:
> 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.

As Jens replied, you mixed the ctx from io uring and blk-mq, both are in
two worlds.

Any wait in this percpu kthread should just wait for generic resource,
not directly related with blk-mq's inactive hctx. Once this thread is
migrated to other online cpu, it will move on.

Thanks,
Ming