netdev - Re: [PATCH] net/flow: remove sleeping and deferral mechanism from flow_cache

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Date:	Thu, 29 Sep 2011 09:24:05 -0400
From:	Benjamin Poirier <benjamin.poirier@...il.com>
To:	Madalin Bucur <madalin.bucur@...escale.com>
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org,
	davem@...emloft.net, timo.teras@....fi
Subject: Re: [PATCH] net/flow: remove sleeping and deferral mechanism from
 flow_cache_flush

On 11-09-26 20:09, Madalin Bucur wrote:
> flow_cache_flush must not sleep as it can be called in atomic context;
> removed the schedule_work as the deferred processing lead to the flow
> cache gc never being actually run under heavy network load
> 
> Signed-off-by: Madalin Bucur <madalin.bucur@...escale.com>
> ---
>  net/core/flow.c |   21 +++++++++------------
>  1 files changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/net/core/flow.c b/net/core/flow.c
> index 555a456..0950f97 100644
> --- a/net/core/flow.c
> +++ b/net/core/flow.c
> @@ -14,7 +14,6 @@
>  #include <linux/init.h>
>  #include <linux/slab.h>
>  #include <linux/smp.h>
> -#include <linux/completion.h>
>  #include <linux/percpu.h>
>  #include <linux/bitops.h>
>  #include <linux/notifier.h>
> @@ -49,7 +48,6 @@ struct flow_cache_percpu {
>  struct flow_flush_info {
>  	struct flow_cache		*cache;
>  	atomic_t			cpuleft;
> -	struct completion		completion;
>  };
>  
>  struct flow_cache {
> @@ -100,7 +98,7 @@ static void flow_entry_kill(struct flow_cache_entry *fle)
>  	kmem_cache_free(flow_cachep, fle);
>  }
>  
> -static void flow_cache_gc_task(struct work_struct *work)
> +static void flow_cache_gc_task(void)
>  {
>  	struct list_head gc_list;
>  	struct flow_cache_entry *fce, *n;
> @@ -113,7 +111,6 @@ static void flow_cache_gc_task(struct work_struct *work)
>  	list_for_each_entry_safe(fce, n, &gc_list, u.gc_list)
>  		flow_entry_kill(fce);
>  }
> -static DECLARE_WORK(flow_cache_gc_work, flow_cache_gc_task);
>  
>  static void flow_cache_queue_garbage(struct flow_cache_percpu *fcp,
>  				     int deleted, struct list_head *gc_list)
> @@ -123,7 +120,7 @@ static void flow_cache_queue_garbage(struct flow_cache_percpu *fcp,
>  		spin_lock_bh(&flow_cache_gc_lock);
>  		list_splice_tail(gc_list, &flow_cache_gc_list);
>  		spin_unlock_bh(&flow_cache_gc_lock);
> -		schedule_work(&flow_cache_gc_work);
> +		flow_cache_gc_task();
>  	}
>  }
>  
> @@ -320,8 +317,7 @@ static void flow_cache_flush_tasklet(unsigned long data)
>  
>  	flow_cache_queue_garbage(fcp, deleted, &gc_list);
>  
> -	if (atomic_dec_and_test(&info->cpuleft))
> -		complete(&info->completion);
> +	atomic_dec(&info->cpuleft);
>  }
>  
>  static void flow_cache_flush_per_cpu(void *data)
> @@ -339,22 +335,23 @@ static void flow_cache_flush_per_cpu(void *data)
>  void flow_cache_flush(void)
>  {
>  	struct flow_flush_info info;
> -	static DEFINE_MUTEX(flow_flush_sem);
> +	static DEFINE_SPINLOCK(flow_flush_lock);
>  
>  	/* Don't want cpus going down or up during this. */
>  	get_online_cpus();
> -	mutex_lock(&flow_flush_sem);
> +	spin_lock_bh(&flow_flush_lock);
>  	info.cache = &flow_cache_global;
>  	atomic_set(&info.cpuleft, num_online_cpus());
> -	init_completion(&info.completion);
>  
>  	local_bh_disable();
local_bh_disable may as well be removed with the change to
spin_lock_bh() just above.

Also, I fail to see why bh_disable is needed for
flow_cache_flush_tasklet(). If you don't mind enlightening me, it'll be
appreciated.

Thanks,
-Ben
>  	smp_call_function(flow_cache_flush_per_cpu, &info, 0);
>  	flow_cache_flush_tasklet((unsigned long)&info);
>  	local_bh_enable();
>  
> -	wait_for_completion(&info.completion);
> -	mutex_unlock(&flow_flush_sem);
> +	while (atomic_read(&info.cpuleft) != 0)
> +		cpu_relax();
> +
> +	spin_unlock_bh(&flow_flush_lock);
>  	put_online_cpus();
>  }
>  
> -- 
> 1.7.0.1
> 
> 
> --
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