From: Pavel Begunkov <asml.silence@gmail.com>
To: John Garry <john.garry@huawei.com>, Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, Omar Sandoval <osandov@osandov.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/4] sbitmap: remove swap_lock
Date: Tue, 24 Nov 2020 14:43:29 +0000 [thread overview]
Message-ID: <5f10de12-ffe1-dfef-d9ff-74b13a7f0426@gmail.com> (raw)
In-Reply-To: <17b6011c-b519-3332-e9b7-de36109db85a@huawei.com>
On 24/11/2020 14:22, John Garry wrote:
> 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.
Yeah, but this part is legit and I don't expect it to be so
contended to need an additional check, especially with atomic
and from [3/4].
I'm more concerned about sbitmap_resize*() callers to do right
synchronisation (e.g. quiesce) and not rely on that critical
section I remove. Would be great if anyone can confirm that.
>
> 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;
>> }
>>
>
--
Pavel Begunkov
next prev parent reply other threads:[~2020-11-24 14:47 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-22 15:35 [PATCH v2 for-next 0/4] optimise sbitmap deferred clear Pavel Begunkov
2020-11-22 15:35 ` [PATCH v2 1/4] sbitmap: optimise sbitmap_deferred_clear() Pavel Begunkov
2020-11-24 14:11 ` John Garry
2020-11-24 15:01 ` Pavel Begunkov
2020-11-22 15:35 ` [PATCH v2 2/4] sbitmap: remove swap_lock Pavel Begunkov
2020-11-24 14:22 ` John Garry
2020-11-24 14:43 ` Pavel Begunkov [this message]
2020-11-26 2:46 ` Ming Lei
2020-11-26 13:44 ` Pavel Begunkov
2020-11-27 2:06 ` Ming Lei
2020-11-22 15:35 ` [PATCH v2 3/4] sbitmap: replace CAS with atomic and Pavel Begunkov
2020-11-22 15:35 ` [PATCH v2 4/4] sbitmap: simplify wrap check Pavel Begunkov
2020-12-08 0:13 ` [PATCH v2 for-next 0/4] optimise sbitmap deferred clear Jens Axboe
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