[PATCH RESEND] block: annotate refault stalls from IO submission

[Johannes Weiner]

psi tracks the time tasks wait for refaulting pages to become
uptodate, but it does not track the time spent submitting the IO. The
submission part can be significant if backing storage is contended or
when cgroup throttling (io.latency) is in effect - a lot of time is
spent in submit_bio(). In that case, we underreport memory pressure.

Annotate submit_bio() to account submission time as memory stall when
the bio is reading userspace workingset pages.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 block/bio.c               |  3 +++
 block/blk-core.c          | 23 ++++++++++++++++++++++-
 include/linux/blk_types.h |  1 +
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index 299a0e7651ec..4196865dd300 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -806,6 +806,9 @@ void __bio_add_page(struct bio *bio, struct page *page,
 
 	bio->bi_iter.bi_size += len;
 	bio->bi_vcnt++;
+
+	if (!bio_flagged(bio, BIO_WORKINGSET) && unlikely(PageWorkingset(page)))
+		bio_set_flag(bio, BIO_WORKINGSET);
 }
 EXPORT_SYMBOL_GPL(__bio_add_page);
 
diff --git a/block/blk-core.c b/block/blk-core.c
index d0cc6e14d2f0..1b1705b7dde7 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -36,6 +36,7 @@
 #include <linux/blk-cgroup.h>
 #include <linux/debugfs.h>
 #include <linux/bpf.h>
+#include <linux/psi.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/block.h>
@@ -1128,6 +1129,10 @@ EXPORT_SYMBOL_GPL(direct_make_request);
  */
 blk_qc_t submit_bio(struct bio *bio)
 {
+	bool workingset_read = false;
+	unsigned long pflags;
+	blk_qc_t ret;
+
 	if (blkcg_punt_bio_submit(bio))
 		return BLK_QC_T_NONE;
 
@@ -1146,6 +1151,8 @@ blk_qc_t submit_bio(struct bio *bio)
 		if (op_is_write(bio_op(bio))) {
 			count_vm_events(PGPGOUT, count);
 		} else {
+			if (bio_flagged(bio, BIO_WORKINGSET))
+				workingset_read = true;
 			task_io_account_read(bio->bi_iter.bi_size);
 			count_vm_events(PGPGIN, count);
 		}
@@ -1160,7 +1167,21 @@ blk_qc_t submit_bio(struct bio *bio)
 		}
 	}
 
-	return generic_make_request(bio);
+	/*
+	 * If we're reading data that is part of the userspace
+	 * workingset, count submission time as memory stall. When the
+	 * device is congested, or the submitting cgroup IO-throttled,
+	 * submission can be a significant part of overall IO time.
+	 */
+	if (workingset_read)
+		psi_memstall_enter(&pflags);
+
+	ret = generic_make_request(bio);
+
+	if (workingset_read)
+		psi_memstall_leave(&pflags);
+
+	return ret;
 }
 EXPORT_SYMBOL(submit_bio);
 
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 1b1fa1557e68..a9dadfc16a92 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -209,6 +209,7 @@ enum {
 	BIO_BOUNCED,		/* bio is a bounce bio */
 	BIO_USER_MAPPED,	/* contains user pages */
 	BIO_NULL_MAPPED,	/* contains invalid user pages */
+	BIO_WORKINGSET,		/* contains userspace workingset pages */
 	BIO_QUIET,		/* Make BIO Quiet */
 	BIO_CHAIN,		/* chained bio, ->bi_remaining in effect */
 	BIO_REFFED,		/* bio has elevated ->bi_cnt */
-- 
2.22.0