Re: [RFC][PATCH] mm: couple rcu and memory reclaim

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Peter Zijlstra wrote:
> 
> Just an idea I had, it seems like a good idea to wait for RCU callbacks
> in reclaim so that we won't get all of memory stuck there.
> 
> If this location is too aggressive we might stick it next to
> disable_swap_token().
> 
> ---
> Couple RCU and reclaim.
> 
> There could be a lot of memory stuck in RCU callbacks. Wait for RCU to
> finish before giving it another go.
> 
> Placed in kswapd and not direct reclaim path because kswapd never holds
> rcu_read_lock() at this point and can thus not deadlock. Direct reclaim
> callers might hold rcu_read_lock() and would suffer from deadlocks if
> sync_rcu() were to be called.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  mm/vmscan.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/mm/vmscan.c
> ===================================================================
> --- linux-2.6.orig/mm/vmscan.c
> +++ linux-2.6/mm/vmscan.c
> @@ -1435,8 +1435,10 @@ loop_again:
>  		unsigned long lru_pages = 0;
> 
>  		/* The swap token gets in the way of swapout... */
> -		if (!priority)
> +		if (!priority) {
> +			synchronize_rcu();

Interesting change

1. I suspect that synchronize_rcu() is most likely to free up
   slab pages, so shrink_slab() will clean up all the freed
   pages. Could we add a comment to indicate the same?
2. Shouldn't we do this in balance_pgdat() as well?


>  			disable_swap_token();
> +		}
> 
>  		all_zones_ok = 1;
> 
> -
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-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL