Re: [PATCH v2] io-wq: handle hashed writes in chains

[Pavel Begunkov <asml.silence@gmail.com>]

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On 19/03/2020 21:56, Jens Axboe wrote:

>  			if (hash != -1U) {
> +				/*
> +				 * If the local list is non-empty, then we
> +				 * have work that hashed to the same key.
> +				 * No need for a lock round-trip, or fiddling
> +				 * the the free/busy state of the worker, or
> +				 * clearing the hashed state. Just process the
> +				 * next one.
> +				 */
> +				if (!work) {
> +					work = wq_first_entry(&list,
> +							      struct io_wq_work,
> +							      list);
> +					if (work) {
> +						wq_node_del(&list, &work->list);
> +						goto got_work;
> +					}
> +				}
> +
>  				spin_lock_irq(&wqe->lock);
>  				wqe->hash_map &= ~BIT_ULL(hash);
>  				wqe->flags &= ~IO_WQE_FLAG_STALLED;

Let's have an example, where "->" is a link
req0(hash=0) -> req1(not_hashed)
req2(hash=0)

1. it grabs @req0 (@work=@req0) and @req1 (in the local @list)
2. it do @req0->func(), sets @work=@req1 and goes to the hash updating code (see
above).

3. ```if (!work)``` check  fails, and it clears @hash_map, even though there is
one of the same hash in the list. It messes up @hash_map accounting, but
probably even can continue working fine.

4. Next, it goes for the second iteration (work == req1), do ->func().
Then checks @hash, which is -1 for non-hashed req1, and exits leaving req2 in
the @list.

Can miss something by looking at diff only, but there are 2 potential points to
fix.

BTW, yours patch executes all linked requests first and then goes to the next
hashed. Mine do hashed first, and re-enqueue linked requests. The downside in my
version is the extra re-enqueue. And your approach can't do some cases in
parallel, e.g. the following is purely sequential:

req0(hash0) -> ... long link ->
req1(hash0) -> ... long link ->
req2(hash0) -> ... long link ->
req3(hash0) -> ... long link ->

It's not hard to convert, though

-- 
Pavel Begunkov


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