Messages in this thread |  | | Subject | Re: [PATCH v2 2/4] sbitmap: remove swap_lock | From | John Garry <> | Date | Tue, 24 Nov 2020 14:22:45 +0000 |
| |
On 22/11/2020 15:35, 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 <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 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;
So if we race with another cpu, won't the 2nd cpu see that the mask is 0 returned from the xchg (not shown)? If so, it's odd to continue to do the CAS - or atomic not, from later patch - on a mask of 0.
Thanks, John
> > /* > * First get a stable cleared mask, setting the old mask to 0. > @@ -35,10 +31,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map) > 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, > @@ -80,7 +73,6 @@ 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; > } >
|  |