public inbox for [email protected]
 help / color / mirror / Atom feed
From: Jens Axboe <[email protected]>
To: "Darrick J. Wong" <[email protected]>
Cc: [email protected], [email protected], [email protected],
	[email protected], [email protected]
Subject: Re: [PATCH 6/8] fs: add IOCB flags related to passing back dio completions
Date: Fri, 21 Jul 2023 09:53:30 -0600	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <20230721154837.GO11352@frogsfrogsfrogs>

On 7/21/23 9:48?AM, Darrick J. Wong wrote:
> On Thu, Jul 20, 2023 at 12:13:08PM -0600, Jens Axboe wrote:
>> Async dio completions generally happen from hard/soft IRQ context, which
>> means that users like iomap may need to defer some of the completion
>> handling to a workqueue. This is less efficient than having the original
>> issuer handle it, like we do for sync IO, and it adds latency to the
>> completions.
>>
>> Add IOCB_DIO_DEFER, which the issuer can set if it is able to safely
>> punt these completions to a safe context. If the dio handler is aware
>> of this flag, assign a callback handler in kiocb->dio_complete and
>> associated data io kiocb->private. The issuer will then call this handler
>> with that data from task context.
>>
>> No functional changes in this patch.
>>
>> Signed-off-by: Jens Axboe <[email protected]>
>> ---
>>  include/linux/fs.h | 34 ++++++++++++++++++++++++++++++++--
>>  1 file changed, 32 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 6867512907d6..2c589418a078 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -338,6 +338,20 @@ enum rw_hint {
>>  #define IOCB_NOIO		(1 << 20)
>>  /* can use bio alloc cache */
>>  #define IOCB_ALLOC_CACHE	(1 << 21)
>> +/*
>> + * IOCB_DIO_DEFER can be set by the iocb owner, to indicate that the
>> + * iocb completion can be passed back to the owner for execution from a safe
>> + * context rather than needing to be punted through a workqueue. If this
>> + * flag is set, the completion handling may set iocb->dio_complete to a
>> + * handler, which the issuer will then call from task context to complete
>> + * the processing of the iocb. iocb->private should then also be set to
>> + * the argument being passed to this handler. Note that while this provides
> 
> Who should be setting iocb->private?  Can I suggest rewording this to:
> 
> "If this flag is set, the bio completion handling may set
> iocb->dio_complete to a handler function and iocb->private to context
> information for that handler.  The issuer should call the handler with
> that context information from task context to complete the processing of
> the iocb."
> 
> Assuming I've understood what this does from the next patch? :)

Yep this is definitely better - thanks, I'll update it!

>> + * a task context for the dio_complete() callback, it should only be used
>> + * on the completion side for non-IO generating completions. It's fine to
>> + * call blocking functions from this callback, but they should not wait for
>> + * unrelated IO (like cache flushing, new IO generation, etc).
>> + */
>> +#define IOCB_DIO_DEFER		(1 << 22)
> 
> Sorry to nitpick names here, but "defer" feels a little vague to me.
> Defer what?  And to whom?
> 
> This flag means "defer iocb completion to the caller", right?  If so,
> wouldn't this be better named IOCB_DIO_CALLER_COMP?

That is probably better indeed. Naming is hard! CALLER_COMP or
ISSUER_COMP would be better and more descriptive. I'll go with your
suggestion.

>>  /* for use in trace events */
>>  #define TRACE_IOCB_STRINGS \
>> @@ -351,7 +365,8 @@ enum rw_hint {
>>  	{ IOCB_WRITE,		"WRITE" }, \
>>  	{ IOCB_WAITQ,		"WAITQ" }, \
>>  	{ IOCB_NOIO,		"NOIO" }, \
>> -	{ IOCB_ALLOC_CACHE,	"ALLOC_CACHE" }
>> +	{ IOCB_ALLOC_CACHE,	"ALLOC_CACHE" }, \
>> +	{ IOCB_DIO_DEFER,	"DIO_DEFER" }
>>  
>>  struct kiocb {
>>  	struct file		*ki_filp;
>> @@ -360,7 +375,22 @@ struct kiocb {
>>  	void			*private;
>>  	int			ki_flags;
>>  	u16			ki_ioprio; /* See linux/ioprio.h */
>> -	struct wait_page_queue	*ki_waitq; /* for async buffered IO */
>> +	union {
>> +		/*
>> +		 * Only used for async buffered reads, where it denotes the
>> +		 * page waitqueue associated with completing the read. Valid
>> +		 * IFF IOCB_WAITQ is set.
>> +		 */
>> +		struct wait_page_queue	*ki_waitq;
>> +		/*
>> +		 * Can be used for O_DIRECT IO, where the completion handling
>> +		 * is punted back to the issuer of the IO. May only be set
>> +		 * if IOCB_DIO_DEFER is set by the issuer, and the issuer must
>> +		 * then check for presence of this handler when ki_complete is
>> +		 * invoked.
> 
> Might want to reiterate in the comment that kiocb.private should be
> passed as @data.

OK, will do.

-- 
Jens Axboe


  reply	other threads:[~2023-07-21 15:54 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-20 18:13 [PATCHSET v4 0/8] Improve async iomap DIO performance Jens Axboe
2023-07-20 18:13 ` [PATCH 1/8] iomap: cleanup up iomap_dio_bio_end_io() Jens Axboe
2023-07-21  6:14   ` Christoph Hellwig
2023-07-21 15:13   ` Darrick J. Wong
2023-07-20 18:13 ` [PATCH 2/8] iomap: add IOMAP_DIO_INLINE_COMP Jens Axboe
2023-07-21  6:14   ` Christoph Hellwig
2023-07-21 15:16   ` Darrick J. Wong
2023-07-20 18:13 ` [PATCH 3/8] iomap: treat a write through cache the same as FUA Jens Axboe
2023-07-21  6:15   ` Christoph Hellwig
2023-07-21 14:04     ` Jens Axboe
2023-07-21 15:55       ` Darrick J. Wong
2023-07-21 16:03         ` Jens Axboe
2023-07-20 18:13 ` [PATCH 4/8] iomap: completed polled IO inline Jens Axboe
2023-07-21  6:16   ` Christoph Hellwig
2023-07-21 15:19   ` Darrick J. Wong
2023-07-21 21:43   ` Dave Chinner
2023-07-22  3:10     ` Jens Axboe
2023-07-22 23:05       ` Dave Chinner
2023-07-24 22:35         ` Jens Axboe
2023-07-22 16:54     ` Jens Axboe
2023-07-20 18:13 ` [PATCH 5/8] iomap: only set iocb->private for polled bio Jens Axboe
2023-07-21  6:18   ` Christoph Hellwig
2023-07-21 15:35   ` Darrick J. Wong
2023-07-21 15:37     ` Jens Axboe
2023-07-20 18:13 ` [PATCH 6/8] fs: add IOCB flags related to passing back dio completions Jens Axboe
2023-07-21  6:18   ` Christoph Hellwig
2023-07-21 15:48   ` Darrick J. Wong
2023-07-21 15:53     ` Jens Axboe [this message]
2023-07-20 18:13 ` [PATCH 7/8] io_uring/rw: add write support for IOCB_DIO_DEFER Jens Axboe
2023-07-21  6:19   ` Christoph Hellwig
2023-07-21 15:50   ` Darrick J. Wong
2023-07-21 15:53     ` Jens Axboe
2023-07-20 18:13 ` [PATCH 8/8] iomap: support IOCB_DIO_DEFER Jens Axboe
2023-07-21  6:19   ` Christoph Hellwig
2023-07-21 16:01   ` Darrick J. Wong
2023-07-21 16:30     ` Jens Axboe
2023-07-21 22:05   ` Dave Chinner
2023-07-22  3:12     ` Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2023-07-24 22:55 [PATCHSET v6 0/8] Improve async iomap DIO performance Jens Axboe
2023-07-24 22:55 ` [PATCH 6/8] fs: add IOCB flags related to passing back dio completions Jens Axboe

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox