* [PATCH for-next 0/2] optimise sbitmap deferred clear
@ 2020-11-21 0:01 Pavel Begunkov
2020-11-21 0:01 ` [PATCH 1/2] sbitmap: optimise sbitmap_deferred_clear() Pavel Begunkov
2020-11-21 0:01 ` [PATCH 2/2] sbitmap: remove swap_lock Pavel Begunkov
0 siblings, 2 replies; 3+ messages in thread
From: Pavel Begunkov @ 2020-11-21 0:01 UTC (permalink / raw)
To: Jens Axboe, linux-block, Omar Sandoval; +Cc: linux-kernel
In short, sbitmap_deferred_clear() lights up pretty much in my profiler,
so these optimisations gave me ~1% more t-put. All the heavy stuff is in
[2/2]. Would love someones eye to check it.
I also want to replace cmpxchg() in that function with a single
atomic and, that's slightly lighter and also transfers it from
lock-free to wait-free, that's pretty neat.
The problem is that apparently there is a non-atomic_t atomic and,
and atomic_t is unsigned int but all the bitmap do unsigned long.
Advice is welcome.
Pavel Begunkov (2):
sbitmap: optimise sbitmap_deferred_clear()
sbitmap: remove swap_lock
include/linux/sbitmap.h | 5 -----
lib/sbitmap.c | 21 +++++++--------------
2 files changed, 7 insertions(+), 19 deletions(-)
--
2.24.0
^ permalink raw reply [flat|nested] 3+ messages in thread
* [PATCH 1/2] sbitmap: optimise sbitmap_deferred_clear()
2020-11-21 0:01 [PATCH for-next 0/2] optimise sbitmap deferred clear Pavel Begunkov
@ 2020-11-21 0:01 ` Pavel Begunkov
2020-11-21 0:01 ` [PATCH 2/2] sbitmap: remove swap_lock Pavel Begunkov
1 sibling, 0 replies; 3+ messages in thread
From: Pavel Begunkov @ 2020-11-21 0:01 UTC (permalink / raw)
To: Jens Axboe, linux-block, Omar Sandoval; +Cc: linux-kernel
Because of spinlocks and atomics sbitmap_deferred_clear() have to reload
&sb->map[index] on each access even though the map address won't change.
Hint it by explicitly caching it in a variable.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
lib/sbitmap.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 267aa7709416..49afb34e8340 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -14,30 +14,31 @@
*/
static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
{
+ struct sbitmap_word *map = &sb->map[index];
unsigned long mask, val;
bool ret = false;
unsigned long flags;
- spin_lock_irqsave(&sb->map[index].swap_lock, flags);
+ spin_lock_irqsave(&map->swap_lock, flags);
- if (!sb->map[index].cleared)
+ if (!map->cleared)
goto out_unlock;
/*
* First get a stable cleared mask, setting the old mask to 0.
*/
- mask = xchg(&sb->map[index].cleared, 0);
+ mask = xchg(&map->cleared, 0);
/*
* Now clear the masked bits in our free word
*/
do {
- val = sb->map[index].word;
- } while (cmpxchg(&sb->map[index].word, val, val & ~mask) != val);
+ val = map->word;
+ } while (cmpxchg(&map->word, val, val & ~mask) != val);
ret = true;
out_unlock:
- spin_unlock_irqrestore(&sb->map[index].swap_lock, flags);
+ spin_unlock_irqrestore(&map->swap_lock, flags);
return ret;
}
--
2.24.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* [PATCH 2/2] sbitmap: remove swap_lock
2020-11-21 0:01 [PATCH for-next 0/2] optimise sbitmap deferred clear Pavel Begunkov
2020-11-21 0:01 ` [PATCH 1/2] sbitmap: optimise sbitmap_deferred_clear() Pavel Begunkov
@ 2020-11-21 0:01 ` Pavel Begunkov
1 sibling, 0 replies; 3+ messages in thread
From: Pavel Begunkov @ 2020-11-21 0:01 UTC (permalink / raw)
To: Jens Axboe, linux-block, Omar Sandoval; +Cc: linux-kernel
map->swap_lock serialises concurrent calls to sbitmap_deferred_clear(),
however that function is already works in atomic fashion and guarantees
to not loose bits while applying map->cleared bitmask.
Remove spinlocking in sbitmap_deferred_clear(). For a one-threaded
tag allocation heavy test on top of nullblk it yields ~1.0-1.5% t-put
increase, and according to perf 3% -> 1.5% cycle reduction of
sbitmap_get().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
include/linux/sbitmap.h | 5 -----
lib/sbitmap.c | 14 +++-----------
2 files changed, 3 insertions(+), 16 deletions(-)
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index e40d019c3d9d..74cc6384715e 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -32,11 +32,6 @@ struct sbitmap_word {
* @cleared: word holding cleared bits
*/
unsigned long cleared ____cacheline_aligned_in_smp;
-
- /**
- * @swap_lock: Held while swapping word <-> cleared
- */
- spinlock_t swap_lock;
} ____cacheline_aligned_in_smp;
/**
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 49afb34e8340..238d9849f24b 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -16,13 +16,9 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
{
struct sbitmap_word *map = &sb->map[index];
unsigned long mask, val;
- bool ret = false;
- unsigned long flags;
- spin_lock_irqsave(&map->swap_lock, flags);
-
- if (!map->cleared)
- goto out_unlock;
+ if (!READ_ONCE(map->cleared))
+ return false;
/*
* First get a stable cleared mask, setting the old mask to 0.
@@ -36,10 +32,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
val = map->word;
} while (cmpxchg(&map->word, val, val & ~mask) != val);
- ret = true;
-out_unlock:
- spin_unlock_irqrestore(&map->swap_lock, flags);
- return ret;
+ return true;
}
int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
@@ -81,7 +74,6 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
for (i = 0; i < sb->map_nr; i++) {
sb->map[i].depth = min(depth, bits_per_word);
depth -= sb->map[i].depth;
- spin_lock_init(&sb->map[i].swap_lock);
}
return 0;
}
--
2.24.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
end of thread, other threads:[~2020-11-21 0:04 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-11-21 0:01 [PATCH for-next 0/2] optimise sbitmap deferred clear Pavel Begunkov
2020-11-21 0:01 ` [PATCH 1/2] sbitmap: optimise sbitmap_deferred_clear() Pavel Begunkov
2020-11-21 0:01 ` [PATCH 2/2] sbitmap: remove swap_lock Pavel Begunkov
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).