Re: [PATCH] psi: annotate refault stalls from IO submission

Johannes Weiner
2019-07-23T20:42:11Z

On Tue, Jul 23, 2019 at 01:34:50PM -0600, Jens Axboe wrote:
> On 7/23/19 1:04 PM, Johannes Weiner wrote:
> > CCing Jens for bio layer stuff
> >
> > On Tue, Jul 23, 2019 at 10:02:26AM +1000, Dave Chinner wrote:
> >> Even better: If this memstall and "refault" check is needed to
> >> account for bio submission blocking, then page cache iteration is
> >> the wrong place to be doing this check. It should be done entirely
> >> in the bio code when adding pages to the bio because we'll only ever
> >> be doing page cache read IO on page cache misses. i.e. this isn't
> >> dependent on adding a new page to the LRU or not - if we add a new
> >> page then we are going to be doing IO and so this does not require
> >> magic pixie dust at the page cache iteration level
> >
> > That could work. I had it at the page cache level because that's
> > logically where the refault occurs. But PG_workingset encodes
> > everything we need from the page cache layer and is available where
> > the actual stall occurs, so we should be able to push it down.
> >
> >> e.g. bio_add_page_memstall() can do the working set check and then
> >> set a flag on the bio to say it contains a memstall page. Then on
> >> submission of the bio the memstall condition can be cleared.
> >
> > A separate bio_add_page_memstall() would have all the problems you
> > pointed out with the original patch: it's magic, people will get it
> > wrong, and it'll be hard to verify and notice regressions.
> >
> > How about just doing it in __bio_add_page()? PG_workingset is not
> > overloaded - when we see it set, we can generally and unconditionally
> > flag the bio as containing userspace workingset pages.
> >
> > At submission time, in conjunction with the IO direction, we can
> > clearly tell whether we are reloading userspace workingset data,
> > i.e. stalling on memory.
> >
> > This?
>
> Not vehemently opposed to it, even if it sucks having to test page flags
> in the hot path.

Yeah, it's not great :/ Just seems marginally better than annotating
all the callsites and maintain correctness there in the future.

> Maybe even do:
>
> if (!bio_flagged(bio, BIO_WORKINGSET) && PageWorkingset(page))
> bio_set_flag(bio, BIO_WORKINGSET);
>
> to at least avoid it for the (common?) case where multiple pages are
> marked as workingset.

Sounds good. If refaults occur, most likely the whole readahead batch
has that flag set, so I've added that. I've also marked the page test
unlikely.

This way we have no jumps in the most common path (no refaults), one
jump in the second most common (bit already set), and the double for
the least likely case of hitting the first refault page in a batch.

Updated patch below.

---
From 1b3888bdf075f86f226af4e350c8a88435d1fe8e Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Thu, 11 Jul 2019 16:01:40 -0400
Subject: [PATCH] psi: annotate refault stalls from IO submission

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@cmpxchg.org>
---
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 29cd6cf4da51..4dd9ea0b068b 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -805,6 +805,9 @@ 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 5d1fc8e17dd1..5993922d63fb 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>
@@ -1127,6 +1128,10 @@ 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 it's a regular read/write or a barrier with data attached,
* go through the normal accounting stuff before submission.
@@ -1142,6 +1147,8 @@ 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);
}
@@ -1156,7 +1163,21 @@ 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 6a53799c3fe2..2f77e3446760 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