Michael et al,
Ok, I redid my failed "hybrid mode" patch from scratch (original
patch never sent out, I never got it to a working point).
Having learnt from my mistake, this time instead of trying to mix the
old and the new code, instead I just extended the new code, and wrote
a _lot_ of comments about it.
I also made it configurable, using a "page_lock_unfairness" knob,
which this patch defaults to 1000 (which is basically infinite).
That's just a value that says how many times we'll try the old unfair
case, so "1000" means "we'll re-queue up to a thousand times before we
say enough is enough" and zero is the fair mode that shows the
performance problems.
I've only (lightly) tested those two extremes, I think the interesting
range is likely in the 1-5 range.
So you can do
echo 0 > /proc/sys/vm/page_lock_unfairness
.. run test ..
and you should get the same numbers as without this patch (within
noise, of course).
Or do
echo 5 > /proc/sys/vm/page_lock_unfairness
.. run test ..
and get numbers for "we accept some unfairness, but if we have to
requeue more than five times, we force the fair mode".
Again, the default is 1000, which is ludicrously high (it's not a
"this many retries per page" count, it's a "for each waiter" count). I
made it that high just because I have *not* run any numbers for that
interesting range, I just checked the extreme cases, and I wanted to
make sure that Michael sees the old performance (modulo other changes
to 5.9, of course).
Comments? The patch really has a fair amount of comments in it, in
fact the code changes are reasonably small, most of the changes really
are about new and updated comments about what is going on.
I was burnt by making a mess of this the first time, so I proceeded
more thoughtfully this time. Hopefullyt the end result is also better.
(Note that it's a commit and has a SHA1, but it's from my "throw-away
tree for testing", so it doesn't have my sign-off or any real commit
message yet: I'll do that once it gets actual testing and comments).
Linus
From 880db10a9fea1dad0c8cf29ae04b4446d2e7170b Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Sun, 13 Sep 2020 14:05:35 -0700
Subject: [PATCH] Page lock unfairness sysctl
---
include/linux/mm.h | 2 +
include/linux/wait.h | 1 +
kernel/sysctl.c | 8 +++
mm/filemap.c | 160 ++++++++++++++++++++++++++++++++++---------
4 files changed, 140 insertions(+), 31 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ca6e6a81576b..b2f370f0b420 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -41,6 +41,8 @@ struct writeback_control;
struct bdi_writeback;
struct pt_regs;
+extern int sysctl_page_lock_unfairness;
+
void init_mm_internals(void);
#ifndef CONFIG_NEED_MULTIPLE_NODES /* Don't use mapnrs, do it properly */
diff --git a/include/linux/wait.h b/include/linux/wait.h
index 898c890fc153..27fb99cfeb02 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -21,6 +21,7 @@ int default_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int
#define WQ_FLAG_WOKEN 0x02
#define WQ_FLAG_BOOKMARK 0x04
#define WQ_FLAG_CUSTOM 0x08
+#define WQ_FLAG_DONE 0x10
/*
* A single wait-queue entry structure:
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 09e70ee2332e..afad085960b8 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2912,6 +2912,14 @@ static struct ctl_table vm_table[] = {
.proc_handler = percpu_pagelist_fraction_sysctl_handler,
.extra1 = SYSCTL_ZERO,
},
+ {
+ .procname = "page_lock_unfairness",
+ .data = &sysctl_page_lock_unfairness,
+ .maxlen = sizeof(sysctl_page_lock_unfairness),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = SYSCTL_ZERO,
+ },
#ifdef CONFIG_MMU
{
.procname = "max_map_count",
diff --git a/mm/filemap.c b/mm/filemap.c
index 1aaea26556cc..d0a76069bcb8 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -988,9 +988,43 @@ void __init pagecache_init(void)
page_writeback_init();
}
+/*
+ * The page wait code treats the "wait->flags" somewhat unusually, because
+ * we have multiple different kinds of waits, not just he usual "exclusive"
+ * one.
+ *
+ * We have:
+ *
+ * (a) no special bits set:
+ *
+ * We're just waiting for the bit to be released, and when a waker
+ * calls the wakeup function, we set WQ_FLAG_WOKEN and wake it up,
+ * and remove it from the wait queue.
+ *
+ * Simple and straightforward.
+ *
+ * (b) WQ_FLAG_EXCLUSIVE:
+ *
+ * The waiter is waiting to get the lock, and only one waiter should
+ * be woken up to avoid any thundering herd behavior. We'll set the
+ * WQ_FLAG_WOKEN bit, wake it up, and remove it from the wait queue.
+ *
+ * This is the traditional exclusive wait.
+ *
+ * (b) WQ_FLAG_EXCLUSIVE | WQ_FLAG_CUSTOM:
+ *
+ * The waiter is waiting to get the bit, and additionally wants the
+ * lock to be transferred to it for fair lock behavior. If the lock
+ * cannot be taken, we stop walking the wait queue without waking
+ * the waiter.
+ *
+ * This is the "fair lock handoff" case, and in addition to setting
+ * WQ_FLAG_WOKEN, we set WQ_FLAG_DONE to let the waiter easily see
+ * that it now has the lock.
+ */
static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync, void *arg)
{
- int ret;
+ unsigned int flags;
struct wait_page_key *key = arg;
struct wait_page_queue *wait_page
= container_of(wait, struct wait_page_queue, wait);
@@ -999,35 +1033,44 @@ static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync,
return 0;
/*
- * If it's an exclusive wait, we get the bit for it, and
- * stop walking if we can't.
- *
- * If it's a non-exclusive wait, then the fact that this
- * wake function was called means that the bit already
- * was cleared, and we don't care if somebody then
- * re-took it.
+ * If it's a lock handoff wait, we get the bit for it, and
+ * stop walking (and do not wake it up) if we can't.
*/
- ret = 0;
- if (wait->flags & WQ_FLAG_EXCLUSIVE) {
- if (test_and_set_bit(key->bit_nr, &key->page->flags))
+ flags = wait->flags;
+ if (flags & WQ_FLAG_EXCLUSIVE) {
+ if (test_bit(key->bit_nr, &key->page->flags))
return -1;
- ret = 1;
+ if (flags & WQ_FLAG_CUSTOM) {
+ if (test_and_set_bit(key->bit_nr, &key->page->flags))
+ return -1;
+ flags |= WQ_FLAG_DONE;
+ }
}
- wait->flags |= WQ_FLAG_WOKEN;
+ /*
+ * We are holding the wait-queue lock, but the waiter that
+ * is waiting for this will be checking the flags without
+ * any locking.
+ *
+ * So update the flags atomically, and wake up the waiter
+ * afterwards to avoid any races. This store-release pairs
+ * with the load-acquire in wait_on_page_bit_common().
+ */
+ smp_store_release(&wait->flags, flags | WQ_FLAG_WOKEN);
wake_up_state(wait->private, mode);
/*
* Ok, we have successfully done what we're waiting for,
* and we can unconditionally remove the wait entry.
*
- * Note that this has to be the absolute last thing we do,
- * since after list_del_init(&wait->entry) the wait entry
+ * Note that this pairs with the "finish_wait()" in the
+ * waiter, and has to be the absolute last thing we do.
+ * After this list_del_init(&wait->entry) the wait entry
* might be de-allocated and the process might even have
* exited.
*/
list_del_init_careful(&wait->entry);
- return ret;
+ return (flags & WQ_FLAG_EXCLUSIVE) != 0;
}
static void wake_up_page_bit(struct page *page, int bit_nr)
@@ -1107,8 +1150,8 @@ enum behavior {
};
/*
- * Attempt to check (or get) the page bit, and mark the
- * waiter woken if successful.
+ * Attempt to check (or get) the page bit, and mark us done
+ * if successful.
*/
static inline bool trylock_page_bit_common(struct page *page, int bit_nr,
struct wait_queue_entry *wait)
@@ -1119,13 +1162,17 @@ static inline bool trylock_page_bit_common(struct page *page, int bit_nr,
} else if (test_bit(bit_nr, &page->flags))
return false;
- wait->flags |= WQ_FLAG_WOKEN;
+ wait->flags |= WQ_FLAG_WOKEN | WQ_FLAG_DONE;
return true;
}
+/* How many times do we accept lock stealing from under a waiter? */
+int sysctl_page_lock_unfairness = 1000;
+
static inline int wait_on_page_bit_common(wait_queue_head_t *q,
struct page *page, int bit_nr, int state, enum behavior behavior)
{
+ int unfairness = sysctl_page_lock_unfairness;
struct wait_page_queue wait_page;
wait_queue_entry_t *wait = &wait_page.wait;
bool thrashing = false;
@@ -1143,11 +1190,18 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
}
init_wait(wait);
- wait->flags = behavior == EXCLUSIVE ? WQ_FLAG_EXCLUSIVE : 0;
wait->func = wake_page_function;
wait_page.page = page;
wait_page.bit_nr = bit_nr;
+repeat:
+ wait->flags = 0;
+ if (behavior == EXCLUSIVE) {
+ wait->flags = WQ_FLAG_EXCLUSIVE;
+ if (--unfairness < 0)
+ wait->flags |= WQ_FLAG_CUSTOM;
+ }
+
/*
* Do one last check whether we can get the
* page bit synchronously.
@@ -1170,27 +1224,63 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
/*
* From now on, all the logic will be based on
- * the WQ_FLAG_WOKEN flag, and the and the page
- * bit testing (and setting) will be - or has
- * already been - done by the wake function.
+ * the WQ_FLAG_WOKEN and WQ_FLAG_DONE flag, to
+ * see whether the page bit testing has already
+ * been done by the wake function.
*
* We can drop our reference to the page.
*/
if (behavior == DROP)
put_page(page);
+ /*
+ * Note that until the "finish_wait()", or until
+ * we see the WQ_FLAG_WOKEN flag, we need to
+ * be very careful with the 'wait->flags', because
+ * we may race with a waker that sets them.
+ */
for (;;) {
+ unsigned int flags;
+
set_current_state(state);
- if (signal_pending_state(state, current))
+ /* Loop until we've been woken or interrupted */
+ flags = smp_load_acquire(&wait->flags);
+ if (!(flags & WQ_FLAG_WOKEN)) {
+ if (signal_pending_state(state, current))
+ break;
+
+ io_schedule();
+ continue;
+ }
+
+ /* If we were non-exclusive, we're done */
+ if (behavior != EXCLUSIVE)
break;
- if (wait->flags & WQ_FLAG_WOKEN)
+ /* If the waker got the lock for us, we're done */
+ if (flags & WQ_FLAG_DONE)
break;
- io_schedule();
+ /*
+ * Otherwise, if we're getting the lock, we need to
+ * try to get it ourselves.
+ *
+ * And if that fails, we'll have to retry this all.
+ */
+ if (unlikely(test_and_set_bit(bit_nr, &page->flags)))
+ goto repeat;
+
+ wait->flags |= WQ_FLAG_DONE;
+ break;
}
+ /*
+ * If a signal happened, this 'finish_wait()' may remove the last
+ * waiter from the wait-queues, but the PageWaiters bit will remain
+ * set. That's ok. The next wakeup will take care of it, and trying
+ * to do it here would be difficult and prone to races.
+ */
finish_wait(q, wait);
if (thrashing) {
@@ -1200,12 +1290,20 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
}
/*
- * A signal could leave PageWaiters set. Clearing it here if
- * !waitqueue_active would be possible (by open-coding finish_wait),
- * but still fail to catch it in the case of wait hash collision. We
- * already can fail to clear wait hash collision cases, so don't
- * bother with signals either.
+ * NOTE! The wait->flags weren't stable until we've done the
+ * 'finish_wait()', and we could have exited the loop above due
+ * to a signal, and had a wakeup event happen after the signal
+ * test but before the 'finish_wait()'.
+ *
+ * So only after the finish_wait() can we reliably determine
+ * if we got woken up or not, so we can now figure out the final
+ * return value based on that state without races.
+ *
+ * Also note that WQ_FLAG_WOKEN is sufficient for a non-exclusive
+ * waiter, but an exclusive one requires WQ_FLAG_DONE.
*/
+ if (behavior == EXCLUSIVE)
+ return wait->flags & WQ_FLAG_DONE ? 0 : -EINTR;
return wait->flags & WQ_FLAG_WOKEN ? 0 : -EINTR;
}
--
2.28.0.218.gc12ef3d349
