Re: [PATCH next v1 2/2] io_uring: limit local tw done

[David Wei <dw@davidwei.uk>]

On 2024-11-20 15:56, Pavel Begunkov wrote:
> On 11/20/24 22:14, David Wei wrote:
>> Instead of eagerly running all available local tw, limit the amount of
>> local tw done to the max of IO_LOCAL_TW_DEFAULT_MAX (20) or wait_nr. The
>> value of 20 is chosen as a reasonable heuristic to allow enough work
>> batching but also keep latency down.
>>
>> Add a retry_llist that maintains a list of local tw that couldn't be
>> done in time. No synchronisation is needed since it is only modified
>> within the task context.
>>
>> Signed-off-by: David Wei <dw@davidwei.uk>
>> ---
>>   include/linux/io_uring_types.h |  1 +
>>   io_uring/io_uring.c            | 43 +++++++++++++++++++++++++---------
>>   io_uring/io_uring.h            |  2 +-
>>   3 files changed, 34 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
>> index 593c10a02144..011860ade268 100644
>> --- a/include/linux/io_uring_types.h
>> +++ b/include/linux/io_uring_types.h
>> @@ -336,6 +336,7 @@ struct io_ring_ctx {
>>        */
>>       struct {
>>           struct llist_head    work_llist;
>> +        struct llist_head    retry_llist;
> 
> Fwiw, probably doesn't matter, but it doesn't even need
> to be atomic, it's queued and spliced while holding
> ->uring_lock, the pending check is also synchronised as
> there is only one possible task doing that.

Yeah, it doesn't. Keeping it as a llist_head means being able to re-use
helpers that take llist_head or llist_node.

> 
>>           unsigned long        check_cq;
>>           atomic_t        cq_wait_nr;
>>           atomic_t        cq_timeouts;
>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>> index 83bf041d2648..c3a7d0197636 100644
>> --- a/io_uring/io_uring.c
>> +++ b/io_uring/io_uring.c
>> @@ -121,6 +121,7 @@
> ...
>>   static int __io_run_local_work(struct io_ring_ctx *ctx, struct io_tw_state *ts,
>>                      int min_events)
>>   {
>>       struct llist_node *node;
>>       unsigned int loops = 0;
>> -    int ret = 0;
>> +    int ret, limit;
>>         if (WARN_ON_ONCE(ctx->submitter_task != current))
>>           return -EEXIST;
>>       if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
>>           atomic_andnot(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
>> +    limit = max(IO_LOCAL_TW_DEFAULT_MAX, min_events);
>>   again:
>> +    ret = __io_run_local_work_loop(&ctx->retry_llist.first, ts, limit);
>> +    if (ctx->retry_llist.first)
>> +        goto retry_done;
>> +
>>       /*
>>        * llists are in reverse order, flip it back the right way before
>>        * running the pending items.
>>        */
>>       node = llist_reverse_order(llist_del_all(&ctx->work_llist));
>> -    while (node) {
>> -        struct llist_node *next = node->next;
>> -        struct io_kiocb *req = container_of(node, struct io_kiocb,
>> -                            io_task_work.node);
>> -        INDIRECT_CALL_2(req->io_task_work.func,
>> -                io_poll_task_func, io_req_rw_complete,
>> -                req, ts);
>> -        ret++;
>> -        node = next;
>> -    }
>> +    ret = __io_run_local_work_loop(&node, ts, ret);
> 
> One thing that is not so nice is that now we have this handling and
> checks in the hot path, and __io_run_local_work_loop() most likely
> gets uninlined.
> 
> I wonder, can we just requeue it via task_work again? We can even
> add a variant efficiently adding a list instead of a single entry,
> i.e. local_task_work_add(head, tail, ...);

That was an early idea, but it means re-reversing the list and then
atomically adding each node back to work_llist concurrently with e.g.
io_req_local_work_add().

Using a separate retry_llist means we don't need to concurrently add to
either retry_llist or work_llist.

> 
> I'm also curious what's the use case you've got that is hitting
> the problem?
> 

There is a Memcache-like workload that has load shedding based on the
time spent doing work. With epoll, the work of reading sockets and
processing a request is done by user, which can decide after some amount
of time to drop the remaining work if it takes too long. With io_uring,
the work of reading sockets is done eagerly inside of task work. If
there is a burst of work, then so much time is spent in task work
reading from sockets that, by the time control returns to user the
timeout has already elapsed.