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

Pavel Begunkov
2020-12-15T11:46:51Z

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