Re: [PATCH 2/2] block: don't allow splitting of a REQ_NOWAIT bio

[Jens Axboe <axboe@kernel.dk>]

On 1/4/23 12:13?PM, Keith Busch wrote:
> On Wed, Jan 04, 2023 at 09:09:38AM -0700, Jens Axboe wrote:
>> If we split a bio marked with REQ_NOWAIT, then we can trigger spurious
>> EAGAIN if constituent parts of that split bio end up failing request
>> allocations. Parts will complete just fine, but just a single failure
>> in one of the chained bios will yield an EAGAIN final result for the
>> parent bio.
>>
>> Return EAGAIN early if we end up needing to split such a bio, which
>> allows for saner recovery handling.
> 
> We're losing some performance here for large-ish single depth IO with
> nvme. We can get a little back by forcing to use the async worker
> earlier in the dispatch instead of getting all the way to the bio
> splitting, but the overhead to check for the condition (which is
> arbitrary decision anyway since we don't know the queue limits at
> io_uring prep time) mostly negates the gain.

Yes, it's not perfect - for perfection, we'd need to be able to either
arbitrarily retry parts of the split bio if we can't get a tag. Or
reserve tags for this request when doing the splits. Either one of those
would require extensive surgery to achieve. In my testing, the cost is
low enough that I think we can live with this for now.

> It's probably fine, though, since you can still hit peak b/w with just a
> little higher qdepth. This patch is a simple way to handle the problem,
> so looks good to me.
> 
> Reviewed-by: Keith Busch <kbusch@kernel.org>

Thanks!

-- 
Jens Axboe