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

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Date:   Thu, 26 Nov 2020 13:44:36 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Omar Sandoval <osandov@...ndov.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] sbitmap: remove swap_lock

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 <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;
> 
> 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