Add RWF_UNCACHED as a read operation flag, which means that any data
read wil be removed from the page cache upon completion. Uses the page
cache to synchronize, and simply prunes folios that were instantiated
when the operation completes. While it would be possible to use private
pages for this, using the page cache as synchronization is handy for a
variety of reasons:
1) No special truncate magic is needed
2) Async buffered reads need some place to serialize, using the page
cache is a lot easier than writing extra code for this
3) The pruning cost is pretty reasonable
and the code to support this is much simpler as a result.
You can think of uncached buffered IO as being the much more attractive
cousing of O_DIRECT - it has none of the restrictions of O_DIRECT. Yes,
it will copy the data, but unlike regular buffered IO, it doesn't run
into the unpredictability of the page cache in terms of reclaim. As an
example, on a test box with 32 drives, reading them with buffered IO
looks as follows:
Reading bs 65536, uncached 0
1s: 145945MB/sec
2s: 158067MB/sec
3s: 157007MB/sec
4s: 148622MB/sec
5s: 118824MB/sec
6s: 70494MB/sec
7s: 41754MB/sec
8s: 90811MB/sec
9s: 92204MB/sec
10s: 95178MB/sec
11s: 95488MB/sec
12s: 95552MB/sec
13s: 96275MB/sec
where it's quite easy to see where the page cache filled up, and
performance went from good to erratic, and finally settles at a much
lower rate. Looking at top while this is ongoing, we see:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7535 root 20 0 267004 0 0 S 3199 0.0 8:40.65 uncached
3326 root 20 0 0 0 0 R 100.0 0.0 0:16.40 kswapd4
3327 root 20 0 0 0 0 R 100.0 0.0 0:17.22 kswapd5
3328 root 20 0 0 0 0 R 100.0 0.0 0:13.29 kswapd6
3332 root 20 0 0 0 0 R 100.0 0.0 0:11.11 kswapd10
3339 root 20 0 0 0 0 R 100.0 0.0 0:16.25 kswapd17
3348 root 20 0 0 0 0 R 100.0 0.0 0:16.40 kswapd26
3343 root 20 0 0 0 0 R 100.0 0.0 0:16.30 kswapd21
3344 root 20 0 0 0 0 R 100.0 0.0 0:11.92 kswapd22
3349 root 20 0 0 0 0 R 100.0 0.0 0:16.28 kswapd27
3352 root 20 0 0 0 0 R 99.7 0.0 0:11.89 kswapd30
3353 root 20 0 0 0 0 R 96.7 0.0 0:16.04 kswapd31
3329 root 20 0 0 0 0 R 96.4 0.0 0:11.41 kswapd7
3345 root 20 0 0 0 0 R 96.4 0.0 0:13.40 kswapd23
3330 root 20 0 0 0 0 S 91.1 0.0 0:08.28 kswapd8
3350 root 20 0 0 0 0 S 86.8 0.0 0:11.13 kswapd28
3325 root 20 0 0 0 0 S 76.3 0.0 0:07.43 kswapd3
3341 root 20 0 0 0 0 S 74.7 0.0 0:08.85 kswapd19
3334 root 20 0 0 0 0 S 71.7 0.0 0:10.04 kswapd12
3351 root 20 0 0 0 0 R 60.5 0.0 0:09.59 kswapd29
3323 root 20 0 0 0 0 R 57.6 0.0 0:11.50 kswapd1
[...]
which is just showing a partial list of the 32 kswapd threads that are
running mostly full tilt, burning ~28 full CPU cores.
If the same test case is run with RWF_UNCACHED set for the buffered read,
the output looks as follows:
Reading bs 65536, uncached 0
1s: 153144MB/sec
2s: 156760MB/sec
3s: 158110MB/sec
4s: 158009MB/sec
5s: 158043MB/sec
6s: 157638MB/sec
7s: 157999MB/sec
8s: 158024MB/sec
9s: 157764MB/sec
10s: 157477MB/sec
11s: 157417MB/sec
12s: 157455MB/sec
13s: 157233MB/sec
14s: 156692MB/sec
which is just chugging along at ~155GB/sec of read performance. Looking
at top, we see:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7961 root 20 0 267004 0 0 S 3180 0.0 5:37.95 uncached
8024 axboe 20 0 14292 4096 0 R 1.0 0.0 0:00.13 top
where just the test app is using CPU, no reclaim is taking place outside
of the main thread. Not only is performance 65% better, it's also using
half the CPU to do it.
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
include/linux/fs.h | 4 +++-
include/uapi/linux/fs.h | 6 +++++-
mm/filemap.c | 18 ++++++++++++++++--
mm/swap.c | 2 ++
4 files changed, 26 insertions(+), 4 deletions(-)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 491eeb73e725..5abc53991cd0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -320,6 +320,7 @@ struct readahead_control;
#define IOCB_NOWAIT (__force int) RWF_NOWAIT
#define IOCB_APPEND (__force int) RWF_APPEND
#define IOCB_ATOMIC (__force int) RWF_ATOMIC
+#define IOCB_UNCACHED (__force int) RWF_UNCACHED
/* non-RWF related bits - start at 16 */
#define IOCB_EVENTFD (1 << 16)
@@ -354,7 +355,8 @@ struct readahead_control;
{ IOCB_SYNC, "SYNC" }, \
{ IOCB_NOWAIT, "NOWAIT" }, \
{ IOCB_APPEND, "APPEND" }, \
- { IOCB_ATOMIC, "ATOMIC"}, \
+ { IOCB_ATOMIC, "ATOMIC" }, \
+ { IOCB_UNCACHED, "UNCACHED" }, \
{ IOCB_EVENTFD, "EVENTFD"}, \
{ IOCB_DIRECT, "DIRECT" }, \
{ IOCB_WRITE, "WRITE" }, \
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 753971770733..dc77cd8ae1a3 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -332,9 +332,13 @@ typedef int __bitwise __kernel_rwf_t;
/* Atomic Write */
#define RWF_ATOMIC ((__force __kernel_rwf_t)0x00000040)
+/* buffered IO that drops the cache after reading or writing data */
+#define RWF_UNCACHED ((__force __kernel_rwf_t)0x00000080)
+
/* mask of flags supported by the kernel */
#define RWF_SUPPORTED (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
- RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC)
+ RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC |\
+ RWF_UNCACHED)
#define PROCFS_IOCTL_MAGIC 'f'
diff --git a/mm/filemap.c b/mm/filemap.c
index 7f8d13f06c04..6f65025782bb 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2471,6 +2471,8 @@ static int filemap_create_folio(struct kiocb *iocb,
folio = filemap_alloc_folio(mapping_gfp_mask(mapping), min_order);
if (!folio)
return -ENOMEM;
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ folio_set_uncached(folio);
/*
* Protect against truncate / hole punch. Grabbing invalidate_lock
@@ -2516,6 +2518,8 @@ static int filemap_readahead(struct kiocb *iocb, struct file *file,
if (iocb->ki_flags & IOCB_NOIO)
return -EAGAIN;
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ ractl.uncached = 1;
page_cache_async_ra(&ractl, folio, last_index - folio->index);
return 0;
}
@@ -2545,6 +2549,8 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
return -EAGAIN;
if (iocb->ki_flags & IOCB_NOWAIT)
flags = memalloc_noio_save();
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ ractl.uncached = 1;
page_cache_sync_ra(&ractl, last_index - index);
if (iocb->ki_flags & IOCB_NOWAIT)
memalloc_noio_restore(flags);
@@ -2705,8 +2711,16 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
}
}
put_folios:
- for (i = 0; i < folio_batch_count(&fbatch); i++)
- folio_put(fbatch.folios[i]);
+ for (i = 0; i < folio_batch_count(&fbatch); i++) {
+ struct folio *folio = fbatch.folios[i];
+
+ if (folio_test_uncached(folio)) {
+ folio_lock(folio);
+ invalidate_complete_folio2(mapping, folio, 0);
+ folio_unlock(folio);
+ }
+ folio_put(folio);
+ }
folio_batch_init(&fbatch);
} while (iov_iter_count(iter) && iocb->ki_pos < isize && !error);
diff --git a/mm/swap.c b/mm/swap.c
index 835bdf324b76..f2457acae383 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -472,6 +472,8 @@ static void folio_inc_refs(struct folio *folio)
*/
void folio_mark_accessed(struct folio *folio)
{
+ if (folio_test_uncached(folio))
+ return;
if (lru_gen_enabled()) {
folio_inc_refs(folio);
return;
--
2.45.2
