Re: [PATCH for-next 5/7] io_uring: add IORING_SETUP_DEFER_TASKRUN

[Dylan Yudaken <dylany@fb.com>]

On Mon, 2022-08-15 at 15:02 +0100, Pavel Begunkov wrote:
> On 8/15/22 14:09, Dylan Yudaken wrote:
> > Allow deferring async tasks until the user calls io_uring_enter(2)
> > with
> > the IORING_ENTER_GETEVENTS flag.
> > 
> > Being able to hand pick when tasks are run prevents the problem
> > where
> > there is current work to be done, however task work runs anyway.
> > 
> > For example, a common workload would obtain a batch of CQEs, and
> > process
> > each one. Interrupting this to additional taskwork would add
> > latency but
> > not gain anything. If instead task work is deferred to just before
> > more
> > CQEs are obtained then no additional latency is added.
> > 
> > The way this is implemented is by trying to keep task work local to
> > a
> > io_ring_ctx, rather than to the submission task. This is required,
> > as the
> > application will want to wake up only a single io_ring_ctx at a
> > time to
> > process work, and so the lists of work have to be kept separate.
> > 
> > This has some other benefits like not having to check the task
> > continually
> > in handle_tw_list (and potentially unlocking/locking those), and
> > reducing
> > locks in the submit & process completions path.
> > 
> > There are networking cases where using this option can reduce
> > request
> > latency by 50%. For example a contrived example using [1] where the
> > client
> > sends 2k data and receives the same data back while doing some
> > system
> > calls (to trigger task work) shows this reduction. The reason ends
> > up
> > being that if sending responses is delayed by processing task work,
> > then
> > the client side sits idle. Whereas reordering the sends first means
> > that
> > the client runs it's workload in parallel with the local task work.
> > 
> > [1]:
> > Using https://github.com/DylanZA/netbench/tree/defer_run
> > Client:
> > ./netbench  --client_only 1 --control_port 10000 --host <host> --tx
> > "epoll --threads 16 --per_thread 1 --size 2048 --resp 2048 --
> > workload 1000"
> > Server:
> > ./netbench  --server_only 1 --control_port 10000  --rx "io_uring --
> > defer_taskrun 0 --workload 100"   --rx "io_uring  --defer_taskrun 1
> > --workload 100"
> > 
> > Signed-off-by: Dylan Yudaken <dylany@fb.com>
> > ---
> >   include/linux/io_uring_types.h |   2 +
> >   include/uapi/linux/io_uring.h  |   7 ++
> >   io_uring/cancel.c              |   2 +-
> >   io_uring/io_uring.c            | 125
> > ++++++++++++++++++++++++++++-----
> >   io_uring/io_uring.h            |  31 +++++++-
> >   io_uring/rsrc.c                |   2 +-
> >   6 files changed, 148 insertions(+), 21 deletions(-)
> > 
> [...]
> > 
> > +
> > +bool io_run_local_work(struct io_ring_ctx *ctx, bool locked)
> > +{
> > +       struct llist_node *node;
> > +       struct llist_node fake;
> > +       struct llist_node *current_final = NULL;
> > +       unsigned int count;
> > +
> > +       if (!locked)
> > +               locked = mutex_trylock(&ctx->uring_lock);
> > +
> > +       node = io_llist_xchg(&ctx->work_llist, &fake);
> > +       count = 0;
> > +again:
> > +       while (node != current_final) {
> > +               struct llist_node *next = node->next;
> > +               struct io_kiocb *req = container_of(node, struct
> > io_kiocb,
> > +                                                  
> > io_task_work.node);
> > +               prefetch(container_of(next, struct io_kiocb,
> > io_task_work.node));
> > +               if (unlikely(!same_thread_group(req->task,
> > current))) {
> 
> Not same thread group, they have to be executed by the same thread.
> One of the assumptions is that current->io_uring is the same
> as the request was initialised with.

How do the wq paths work in that case? I can see in io_queue_iowq that
we only check for same_thread_group.

If required to be the same task we'd probably want to enforce
IORING_SETUP_SINGLE_ISSUER for this flag (not a big problem).

> 
> > +                       __io_req_task_work_add(req, false);
> 
> Why do we mix local and normal tw in the same ring? I think we
> need to do either one or another. What is blocking it?

This is for some slow paths such as cancelling requests or the ring
shutting down. We need to make sure things are run and so need to have
both ways.

> 
> > +               } else {
> > +                       req->io_task_work.func(req, &locked);
> > +                       count++;
> > +               }
> > +               node = next;
> > +       }
> > +
> > +       if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
> > +               atomic_andnot(IORING_SQ_TASKRUN, &ctx->rings-
> > >sq_flags);
> > +
> > +       node = io_llist_cmpxchg(&ctx->work_llist, &fake, NULL);
> > +       if (node != &fake) {
> > +               current_final = &fake;
> > +               node = io_llist_xchg(&ctx->work_llist, &fake);
> > +               goto again;
> > +       }
> > +
> > +       if (locked) {
> > +               io_submit_flush_completions(ctx);
> > +               mutex_unlock(&ctx->uring_lock);
> > +       }
> > +       return count > 0;
> > +}
> > +
> >   static void io_req_tw_post(struct io_kiocb *req, bool *locked)
> >   {
> >         io_req_complete_post(req);
> > @@ -1284,8 +1366,7 @@ static int io_iopoll_check(struct io_ring_ctx
> > *ctx, long min)
> >                 if (wq_list_empty(&ctx->iopoll_list)) {
> >                         u32 tail = ctx->cached_cq_tail;
> >   
> > -                       mutex_unlock(&ctx->uring_lock);
> > -                       io_run_task_work();
> > +                       io_run_task_work_unlock_ctx(ctx);
> >                         mutex_lock(&ctx->uring_lock);
> >   
> >                         /* some requests don't go through
> > iopoll_list */
> > @@ -2146,7 +2227,9 @@ struct io_wait_queue {
> >   
> >   static inline bool io_has_work(struct io_ring_ctx *ctx)
> >   {
> > -       return test_bit(IO_CHECK_CQ_OVERFLOW_BIT, &ctx->check_cq);
> > +       return test_bit(IO_CHECK_CQ_OVERFLOW_BIT, &ctx->check_cq)
> > ||
> > +              ((ctx->flags & IORING_SETUP_DEFER_TASKRUN) &&
> > +               !llist_empty(&ctx->work_llist));
> >   }
> >   
> >   static inline bool io_should_wake(struct io_wait_queue *iowq)
> > @@ -2178,9 +2261,9 @@ static int io_wake_function(struct
> > wait_queue_entry *curr, unsigned int mode,
> >         return -1;
> >   }
> >   
> > -int io_run_task_work_sig(void)
> > +int io_run_task_work_sig(struct io_ring_ctx *ctx)
> >   {
> > -       if (io_run_task_work())
> > +       if (io_run_task_work_ctx(ctx, true))
> >                 return 1;
> >         if (task_sigpending(current))
> >                 return -EINTR;
> > @@ -2196,7 +2279,7 @@ static inline int
> > io_cqring_wait_schedule(struct io_ring_ctx *ctx,
> >         unsigned long check_cq;
> >   
> >         /* make sure we run task_work before checking for signals
> > */
> > -       ret = io_run_task_work_sig();
> > +       ret = io_run_task_work_sig(ctx);
> >         if (ret || io_should_wake(iowq))
> >                 return ret;
> >   
> > @@ -2230,7 +2313,7 @@ static int io_cqring_wait(struct io_ring_ctx
> > *ctx, int min_events,
> >                 io_cqring_overflow_flush(ctx);
> >                 if (io_cqring_events(ctx) >= min_events)
> >                         return 0;
> > -               if (!io_run_task_work())
> > +               if (!io_run_task_work_ctx(ctx, false))
> >                         break;
> >         } while (1);
> >   
> > @@ -2768,13 +2851,14 @@ static __cold bool
> > io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
> >                 }
> >         }
> >   
> > +       io_move_task_work_from_local(ctx);
> 
> Why do we even need to move them? Isn't it easier to just
> execute them here?

I think I was being too conservative. Can execute them here, and if it
is the wrong task can move them to that task's task_work.
> 
> >         ret |= io_cancel_defer_files(ctx, task, cancel_all);
> >         mutex_lock(&ctx->uring_lock);
> >         ret |= io_poll_remove_all(ctx, task, cancel_all);
> >         mutex_unlock(&ctx->uring_lock);
> >         ret |= io_kill_timeouts(ctx, task, cancel_all);
> >         if (task)
> > -               ret |= io_run_task_work();
> > +               ret |= io_run_task_work_ctx(ctx, true);
> >         return ret;
> >   }
> >   
> > @@ -2837,7 +2921,7 @@ __cold void io_uring_cancel_generic(bool
> > cancel_all, struct io_sq_data *sqd)
> >                 }
> >   
> >                 prepare_to_wait(&tctx->wait, &wait,
> > TASK_INTERRUPTIBLE);
> > -               io_run_task_work();
> > +               io_run_task_work_ctx(ctx, true);
> >                 io_uring_drop_tctx_refs(current);
> >   
> >                 /*
> > @@ -3055,12 +3139,15 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned
> > int, fd, u32, to_submit,
> >                         mutex_unlock(&ctx->uring_lock);
> >                         goto out;
> >                 }
> > -               if ((flags & IORING_ENTER_GETEVENTS) && ctx-
> > >syscall_iopoll)
> > +
> > +               if (!(flags & IORING_ENTER_GETEVENTS))
> > +                       mutex_unlock(&ctx->uring_lock);
> > +               else if (ctx->syscall_iopoll)
> >                         goto iopoll_locked;
> > -               mutex_unlock(&ctx->uring_lock);
> > -               io_run_task_work();
> > +               else
> > +                       io_run_task_work_unlock_ctx(ctx);
> 
> Let's unroll this function and get rid of conditional
> locking, especially since you don't need the io_run_task_work()
> part here.
> 
> >         } else {
> > -               io_run_task_work();
> > +               io_run_task_work_ctx(ctx, false);
> >         }
> >   
> ...
> > --- a/io_uring/io_uring.h
> > +++ b/io_uring/io_uring.h
> > @@ -26,7 +26,8 @@ enum {
> >   
> ...
> >   
> > +static inline bool io_run_task_work_ctx(struct io_ring_ctx *ctx,
> > bool all)
> > +{
> > +       bool ret = false;
> > +
> > +       if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
> > +               ret = io_run_local_work(ctx, false);
> > +               if (!all)
> > +                       return ret;
> > +       }
> > +       ret  |= io_run_task_work();
> > +       return ret;
> > +}
> > +
> > +static inline bool io_run_task_work_unlock_ctx(struct io_ring_ctx
> > *ctx)
> > +{
> > +       bool ret;
> > +
> > +       if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
> > +               ret = io_run_local_work(ctx, true);
> > +               mutex_unlock(&ctx->uring_lock);
> 
> If I read it right, io_run_local_work(locked=true) will
> release the lock, and then we unlock it a second time
> here. I'd suggest moving conditional locking out
> of io_run_local_work().

Ah - thanks for spotting that. I'll clean it up