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

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Date:   Tue, 24 Nov 2020 14:22:45 +0000
From:   John Garry <john.garry@...wei.com>
To:     Pavel Begunkov <asml.silence@...il.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 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.

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;
>   }
>