Re: Improving AIO cancellation

[Bart Van Assche]

On 02/08/13 04:42, Anatol Pomozov wrote:
> Hi,
>
> At Google we have several applications that heavily use asynchronous
> IO. One thing that our userspace developers need is effective AIO
> cancellation. You might say "sure use io_cancel syscall". Well, while
> it cancels AIO requests it does it ineffectively. Currently (I am
> looking at linux-next) io_cancel only marks kiocb as cancelled. The
> bios still be issued to device even after kiocb was cancelled. Let's
> say you have a congested device and want to cancel some AIO requests -
> io_cancel will not make situation better. We would like to see more
> resource effective AIO cancellation.
>
> I had a discussion with Ted Tso and Kent Overstreet about improving
> this situation and would like to share the ideas with you, linux
> community.
>
> Once direct async IO is submitted the request can be at several stages:
> 1) Sitting in kernel request queue of a congested device
> 2) Sent to device and sitting in device queue (if NCQ is enabled)
> 3) Executing on device
>
> Ideally if we can cancel an IO request on any of these stages. But
> currently we are especially interested in case #1. I do not know if
> cancellation at stage #2 and #3 is possible and/or reasonable.
>
> BTW AIO cancellation makes sense only for direct IO. Buffered AIO will
> end up in buffer soon and kiocb will be marked as completed. Later
> (maybe much later) writeback will flush those buffers to disk, but you
> cannot cancel it..
>
> And yet another thing to remember is md/RAID. Some types of raid
> support stripes consistency. When md splits a WRITE across disks
> either all or no of the child requests should be completed. If we do
> partial write then the disk data will become inconsistent.
>
>
> Ted and Kent suggested following solution: any time when we do forward
> progress with request/bio we need to check its status. If user
> cancelled the request then just skip this bio. So it covers case #1.
>
> The draft implementation will look like this. struct bio should have
> some way to get current status of kiocb that generated bio. So we add
> a pointer to bool flag.
>
> struct bio {
>    bool *cancelled;
> }
>
> in async DIO codepath this pointer will be initialized with bool at
> "struct kiocb"
>    bio->cancelled = &kiocb->cancelled;
> except md. If it is RAID5 and we perform WRITE request then we do not
> initialize this pointer.
>
>
> when we do forward progress with request/bio we check its cancellation status:
> if (bio->cancelled && *bio->cancelled)
>    goto do_not_process_bio_because_it_cancelled;
>
> So to cancel kiocb we do
> kiocb->cancelled = true;
> and all bio created from the request will not be send to device anymore.
>
>
> The solution seems straightforward, but I would like to hear if there
> are other solutions to make AIO cancellation better. Does suggested
> implementation looks good? Are there better solutions? What about
> cancelling requests that are already sent to device?
>
> If the proposal is fine then I start implementing it.

Hello Anatol,

Had you already noticed this message: 
http://marc.info/?l=linux-fsdevel&m=136024044202362 ?

Bart.

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