Re: [PATCH v1 4/6] block/psi: remove PSI annotations from direct IO

[Pavel Begunkov <asml.silence@gmail.com>]

On 15/12/2020 01:33, Dave Chinner wrote:
> On Tue, Dec 15, 2020 at 01:03:45AM +0000, Pavel Begunkov wrote:
>> On 15/12/2020 00:56, Dave Chinner wrote:
>>> On Tue, Dec 15, 2020 at 12:20:23AM +0000, Pavel Begunkov wrote:
>>>> As reported, we must not do pressure stall information accounting for
>>>> direct IO, because otherwise it tells that it's thrashing a page when
>>>> actually doing IO on hot data.
>>>>
>>>> Apparently, bio_iov_iter_get_pages() is used only by paths doing direct
>>>> IO, so just make it avoid setting BIO_WORKINGSET, it also saves us CPU
>>>> cycles on doing that. For fs/direct-io.c just clear the flag before
>>>> submit_bio(), it's not of much concern performance-wise.
>>>>
>>>> Reported-by: Christoph Hellwig <hch@infradead.org>
>>>> Suggested-by: Christoph Hellwig <hch@infradead.org>
>>>> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
>>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>>> ---
>>>>  block/bio.c    | 25 ++++++++++++++++---------
>>>>  fs/direct-io.c |  2 ++
>>>>  2 files changed, 18 insertions(+), 9 deletions(-)
>>> .....
>>>> @@ -1099,6 +1103,9 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
>>>>   * fit into the bio, or are requested in @iter, whatever is smaller. If
>>>>   * MM encounters an error pinning the requested pages, it stops. Error
>>>>   * is returned only if 0 pages could be pinned.
>>>> + *
>>>> + * It also doesn't set BIO_WORKINGSET, so is intended for direct IO. If used
>>>> + * otherwise the caller is responsible to do that to keep PSI happy.
>>>>   */
>>>>  int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>>>>  {
>>>> diff --git a/fs/direct-io.c b/fs/direct-io.c
>>>> index d53fa92a1ab6..914a7f600ecd 100644
>>>> --- a/fs/direct-io.c
>>>> +++ b/fs/direct-io.c
>>>> @@ -426,6 +426,8 @@ static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
>>>>  	unsigned long flags;
>>>>  
>>>>  	bio->bi_private = dio;
>>>> +	/* PSI is only for paging IO */
>>>> +	bio_clear_flag(bio, BIO_WORKINGSET);
>>>
>>> Why only do this for the old direct IO path? Why isn't this
>>> necessary for the iomap DIO path?
>>
>> It's in the description. In short, block and iomap dio use
>> bio_iov_iter_get_pages(), which with this patch doesn't use
>> [__]bio_add_page() and so doesn't set the flag. 
> 
> That is not obvious to someone not intimately familiar with the
> patchset you are working on. You described -what- the code is doing,
> not -why- the flag needs to be cleared here.

It's missing the link between BIO_WORKINGSET and PSI, but otherwise
it describe both, what it does and how. I'll reword it for you next
iteration.

> 
> "Direct IO does not operate on the current working set of pages
> managed by the kernel, so it should not be accounted as IO to the
> pressure stall tracking infrastructure. Only direct IO paths use
> bio_iov_iter_get_pages() to build bios, so to avoid PSI tracking of
> direct IO don't flag the bio with BIO_WORKINGSET in this function.
> 
> fs/direct-io.c uses <some other function> to build the bio we
> are going to submit and so still flags the bio with BIO_WORKINGSET.
> Rather than convert it to use bio_iov_iter_get_pages() to avoid
> flagging the bio, we simply clear the BIO_WORKINGSET flag before
> submitting the bio."
> 
> Cheers,
> 
> Dave.
> 

-- 
Pavel Begunkov