Re: [PATCH v6 0/6] task work optimization

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/12/3 下午10:21, Pavel Begunkov 写道:
> On 12/3/21 07:30, Hao Xu wrote:
>> 在 2021/12/3 上午10:01, Pavel Begunkov 写道:
>>> On 12/3/21 01:39, Pavel Begunkov wrote:
>>>> On 11/26/21 10:07, Hao Xu wrote:
>>>>> v4->v5
>>>>> - change the implementation of merge_wq_list
>>>>>
> [...]
>>> But testing with liburing tests I'm getting the stuff below,
>>> e.g. cq-overflow hits it every time. Double checked that
>>> I took [RESEND] version of 6/6.
>>>
>>> [   30.360370] BUG: scheduling while atomic: cq-overflow/2082/0x00000000
>>> [   30.360520] Call Trace:
>>> [   30.360523]  <TASK>
>>> [   30.360527]  dump_stack_lvl+0x4c/0x63
>>> [   30.360536]  dump_stack+0x10/0x12
>>> [   30.360540]  __schedule_bug.cold+0x50/0x5e
>>> [   30.360545]  __schedule+0x754/0x900
>>> [   30.360551]  ? __io_cqring_overflow_flush+0xb6/0x200
>>> [   30.360558]  schedule+0x55/0xd0
>>> [   30.360563]  schedule_timeout+0xf8/0x140
>>> [   30.360567]  ? prepare_to_wait_exclusive+0x58/0xa0
>>> [   30.360573]  __x64_sys_io_uring_enter+0x69c/0x8e0
>>> [   30.360578]  ? io_rsrc_buf_put+0x30/0x30
>>> [   30.360582]  do_syscall_64+0x3b/0x80
>>> [   30.360588]  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>> [   30.360592] RIP: 0033:0x7f9f9680118d
>>> [   30.360618]  </TASK>
>>> [   30.362295] BUG: scheduling while atomic: cq-overflow/2082/0x7ffffffe
>>> [   30.362396] Call Trace:
>>> [   30.362397]  <TASK>
>>> [   30.362399]  dump_stack_lvl+0x4c/0x63
>>> [   30.362406]  dump_stack+0x10/0x12
>>> [   30.362409]  __schedule_bug.cold+0x50/0x5e
>>> [   30.362413]  __schedule+0x754/0x900
>>> [   30.362419]  schedule+0x55/0xd0
>>> [   30.362423]  schedule_timeout+0xf8/0x140
>>> [   30.362427]  ? prepare_to_wait_exclusive+0x58/0xa0
>>> [   30.362431]  __x64_sys_io_uring_enter+0x69c/0x8e0
>>> [   30.362437]  ? io_rsrc_buf_put+0x30/0x30
>>> [   30.362440]  do_syscall_64+0x3b/0x80
>>> [   30.362445]  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>> [   30.362449] RIP: 0033:0x7f9f9680118d
>>> [   30.362470]  </TASK>
>>> <repeated>
>>>
>> cannot repro this, all the liburing tests work well on my side..
> 
> One problem is when on the first iteration tctx_task_work doen't
> have anything in prior_task_list, it goes to handle_tw_list(),
> which sets up @ctx but leaves @locked=false (say there is
> contention). And then on the second iteration it goes to
> handle_prior_tw_list() with non-NULL @ctx and @locked=false,
> and tries to unlock not locked spin.
> 
> Not sure that's the exactly the problem from traces, but at
> least a quick hack resetting the ctx at the beginning of
> handle_prior_tw_list() heals it.
Good catch, thanks.
> 
> note: apart from the quick fix the diff below includes
> a couple of lines to force it to go through the new path.
> 
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 66d119ac4424..3868123eef87 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2272,6 +2272,9 @@ static inline void ctx_commit_and_unlock(struct 
> io_ring_ctx *ctx)
>   static void handle_prior_tw_list(struct io_wq_work_node *node, struct 
> io_ring_ctx **ctx,
>                                   bool *locked)
>   {
> +       ctx_flush_and_put(*ctx, locked);
> +       *ctx = NULL;
> +
>          do {
>                  struct io_wq_work_node *next = node->next;
>                  struct io_kiocb *req = container_of(node, struct io_kiocb,
> @@ -2283,7 +2286,8 @@ static void handle_prior_tw_list(struct 
> io_wq_work_node *node, struct io_ring_ct
>                          ctx_flush_and_put(*ctx, locked);
>                          *ctx = req->ctx;
>                          /* if not contended, grab and improve batching */
> -                       *locked = mutex_trylock(&(*ctx)->uring_lock);
> +                       *locked = false;
> +                       // *locked = mutex_trylock(&(*ctx)->uring_lock);
I believe this one is your debug code which I shouldn't take, should I?
>                          percpu_ref_get(&(*ctx)->refs);
>                          if (unlikely(!*locked))
>                                  spin_lock(&(*ctx)->completion_lock);
> @@ -2840,7 +2844,7 @@ static void io_complete_rw(struct kiocb *kiocb, 
> long res)
>                  return;
>          req->result = res;
>          req->io_task_work.func = io_req_task_complete;
> -       io_req_task_work_add(req, !!(req->ctx->flags & 
> IORING_SETUP_SQPOLL));
> +       io_req_task_work_add(req, true);
>   }
> 
> 
>