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

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

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. 

-- 
Pavel Begunkov