Re: [PATCH RFC] Enable bio cache for IRQ driven IO from io_uring

[Jens Axboe <axboe@kernel.dk>]

On 8/19/21 3:01 AM, Christoph Hellwig wrote:
> On Wed, Aug 18, 2021 at 10:54:45AM -0600, Jens Axboe wrote:
>> We previously enabled this for O_DIRECT polled IO, however io_uring
>> completes all IO from task context these days, so it can be enabled for
>> that path too. This requires moving the bio_put() from IRQ context, and
>> this can be accomplished by passing the ownership back to the issuer.
>>
>> Use kiocb->private for that, which should be (as far as I can tell) free
>> once we get to the completion side of things. Add a IOCB_PUT_CACHE flag
>> to tell the issuer that we passed back the ownership, then the issuer
>> can put the bio from a safe context.
>>
>> Like the polled IO ditto, this is good for a 10% performance increase.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> Just hacked this up and tested it, Works For Me. Would welcome input on
>> alternative methods here, if anyone has good suggestions.
> 
> 10% performance improvement looks really nice, but I don't think we can
> just hardcode assumptions about bios in iomap->private.  The easiest
> would be to call back into the file systems for the freeing, but that
> would add an indirect call.

That's why it's an RFC - while it's not the prettiest, the ->ki_complete
assigner is also the one that sets IOCB_ALLOC_CACHE, and hence it's not
that hard to verify that it does IOCB_PUT_CACHE correctly too. That
said, I would prefer a better way of passing the bio back. There are
other optimizations that could be done if we do that. But I have no good
ideas on how to do the passing differently right now.

-- 
Jens Axboe