On 26/11/2020 02:46, Ming Lei wrote: > On Sun, Nov 22, 2020 at 03:35:46PM +0000, Pavel Begunkov wrote: >> map->swap_lock protects map->cleared from concurrent modification, >> however sbitmap_deferred_clear() is already atomically drains it, so >> it's guaranteed to not loose bits on concurrent >> sbitmap_deferred_clear(). >> >> A one threaded tag heavy test on top of nullbk showed ~1.5% t-put >> increase, and 3% -> 1% cycle reduction of sbitmap_get() according to perf. >> >> Signed-off-by: Pavel Begunkov <[email protected]> >> --- >> 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 c1c8a4e69325..4fd877048ba8 100644 >> --- a/lib/sbitmap.c >> +++ b/lib/sbitmap.c >> @@ -15,13 +15,9 @@ >> static inline bool sbitmap_deferred_clear(struct sbitmap_word *map) >> { >> 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; > > This way might break sbitmap_find_bit_in_index()/sbitmap_get_shallow(). > Currently if sbitmap_deferred_clear() returns false, it means nothing > can be allocated from this word. With this patch, even though 'false' > is returned, free bits still might be available because another > sbitmap_deferred_clear() can be run concurrently.
But that can happen anyway if someone frees a requests right after we return from sbitmap_deferred_clear(). Can you elaborate what exactly it breaks? Something in sbq wakeup paths? -- Pavel Begunkov
