linux-kernel - Re: [PATCH v2 2/4] sbitmap: remove swap

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Message-ID: <5f10de12-ffe1-dfef-d9ff-74b13a7f0426@gmail.com>
Date:   Tue, 24 Nov 2020 14:43:29 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     John Garry <john.garry@...wei.com>, Jens Axboe <axboe@...nel.dk>,
        linux-block@...r.kernel.org, Omar Sandoval <osandov@...ndov.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] sbitmap: remove swap_lock

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@...il.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