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Message-ID: <e8adf941-9901-b54c-d7a0-b785923558fb@gmail.com>
Date: Tue, 15 Dec 2020 01:03:45 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Dave Chinner <david@...morbit.com>
Cc: linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
Christoph Hellwig <hch@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
Ming Lei <ming.lei@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
"Darrick J . Wong" <darrick.wong@...cle.com>,
"Martin K . Petersen" <martin.petersen@...cle.com>,
Jonathan Corbet <corbet@....net>, linux-xfs@...r.kernel.org,
linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
linux-kernel@...r.kernel.org, target-devel@...r.kernel.org,
linux-scsi@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v1 4/6] block/psi: remove PSI annotations from direct IO
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@...radead.org>
>> Suggested-by: Christoph Hellwig <hch@...radead.org>
>> Suggested-by: Johannes Weiner <hannes@...xchg.org>
>> Signed-off-by: Pavel Begunkov <asml.silence@...il.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
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