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

On 8/19/21 3:01 AM, Christoph Hellwig wrote:

On Wed, Aug 18, 2021 at 10:54:45AM -0600, Jens Axboe wrote:

quoted

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