[Devel] Re: [PATCH] memory.min_usage (seqlock for res_counter)

[Pavel Emelyanov]

KAMEZAWA Hiroyuki wrote:
> This is seqlock version res_counter.
> Maybe this this will reduce # of spin_lock.
> 
> Pavel-san, How about this ?

AFAIS the readlock is used only in the check_under_limit(),
but I think, that even if we read usage and limit values
in this case non-atomically, this won't result in any 
dramatic sequence at all. No?

On the other hand, we make the charge/uncharge routines
work longer :(

> -Kame
> ==
> resource counter's spinlock can be seqlock. (res_counter doesn't include
> any pointers.)
> 
> And it will be good to avoid atomic ops if we can when we just checks
> value.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> 
> 
> 
> 
>  include/linux/res_counter.h |   24 +++++++-----------------
>  kernel/res_counter.c        |   14 +++++++-------
>  2 files changed, 14 insertions(+), 24 deletions(-)
> 
> Index: linux-2.6.24-rc3-mm2/include/linux/res_counter.h
> ===================================================================
> --- linux-2.6.24-rc3-mm2.orig/include/linux/res_counter.h
> +++ linux-2.6.24-rc3-mm2/include/linux/res_counter.h
> @@ -12,6 +12,7 @@
>   */
>  
>  #include <linux/cgroup.h>
> +#include <linux/seqlock.h>
>  
>  /*
>   * The core object. the cgroup that wishes to account for some
> @@ -36,7 +37,7 @@ struct res_counter {
>  	 * the lock to protect all of the above.
>  	 * the routines below consider this to be IRQ-safe
>  	 */
> -	spinlock_t lock;
> +	seqlock_t lock;
>  };
>  
>  /*
> @@ -101,27 +102,17 @@ int res_counter_charge(struct res_counte
>  void res_counter_uncharge_locked(struct res_counter *counter, unsigned long val);
>  void res_counter_uncharge(struct res_counter *counter, unsigned long val);
>  
> -static inline bool res_counter_limit_check_locked(struct res_counter *cnt)
> -{
> -	if (cnt->usage < cnt->limit)
> -		return true;
> -
> -	return false;
> -}
> -
> -/*
> - * Helper function to detect if the cgroup is within it's limit or
> - * not. It's currently called from cgroup_rss_prepare()
> - */
>  static inline bool res_counter_check_under_limit(struct res_counter *cnt)
>  {
>  	bool ret;
> -	unsigned long flags;
> +	unsigned long seq;
>  
> -	spin_lock_irqsave(&cnt->lock, flags);
> -	ret = res_counter_limit_check_locked(cnt);
> -	spin_unlock_irqrestore(&cnt->lock, flags);
> +	do {
> +		seq = read_seqbegin(&cnt->lock);
> +		ret = (cnt->usage < cnt->limit);
> +	} while (read_seqretry(&cnt->lock, seq));
>  	return ret;
>  }
>  
> +
>  #endif
> Index: linux-2.6.24-rc3-mm2/kernel/res_counter.c
> ===================================================================
> --- linux-2.6.24-rc3-mm2.orig/kernel/res_counter.c
> +++ linux-2.6.24-rc3-mm2/kernel/res_counter.c
> @@ -15,7 +15,7 @@
>  
>  void res_counter_init(struct res_counter *counter)
>  {
> -	spin_lock_init(&counter->lock);
> +	seqlock_init(&counter->lock);
>  	counter->limit = (unsigned long long)LLONG_MAX;
>  }
>  
> @@ -35,9 +35,9 @@ int res_counter_charge(struct res_counte
>  	int ret;
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&counter->lock, flags);
> +	write_seqlock_irqsave(&counter->lock, flags);
>  	ret = res_counter_charge_locked(counter, val);
> -	spin_unlock_irqrestore(&counter->lock, flags);
> +	write_sequnlock_irqrestore(&counter->lock, flags);
>  	return ret;
>  }
>  
> @@ -53,9 +53,9 @@ void res_counter_uncharge(struct res_cou
>  {
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&counter->lock, flags);
> +	write_seqlock_irqsave(&counter->lock, flags);
>  	res_counter_uncharge_locked(counter, val);
> -	spin_unlock_irqrestore(&counter->lock, flags);
> +	write_sequnlock_irqrestore(&counter->lock, flags);
>  }
>  
>  
> @@ -122,10 +122,10 @@ ssize_t res_counter_write(struct res_cou
>  		if (*end != '\0')
>  			goto out_free;
>  	}
> -	spin_lock_irqsave(&counter->lock, flags);
> +	write_seqlock_irqsave(&counter->lock, flags);
>  	val = res_counter_member(counter, member);
>  	*val = tmp;
> -	spin_unlock_irqrestore(&counter->lock, flags);
> +	write_sequnlock_irqrestore(&counter->lock, flags);
>  	ret = nbytes;
>  out_free:
>  	kfree(buf);
> 
> 

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