linux-kernel - Re: [PATCH] vfs: Avoid IPI storm due to bh LRU invalidation

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Date:	Mon, 06 Feb 2012 21:12:36 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>
CC:	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	hare@...e.de, Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Gilad Ben-Yossef <gilad@...yossef.com>
Subject: Re: [PATCH] vfs: Avoid IPI storm due to bh LRU invalidation

On 02/06/2012 07:25 PM, Jan Kara wrote:

> When discovery of lots of disks happen in parallel, we call
> invalidate_bh_lrus() once for each disk from partitioning code resulting in a
> storm of IPIs and causing a softlockup detection to fire (it takes several
> *minutes* for a machine to execute all the invalidate_bh_lrus() calls).
> 
> Fix the issue by allowing only single invalidation to run using a mutex and let
> waiters for mutex figure out whether someone invalidated LRUs for them while
> they were waiting.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/buffer.c |   23 ++++++++++++++++++++++-
>  1 files changed, 22 insertions(+), 1 deletions(-)
> 
>   I feel this is slightly hacky approach but it works. If someone has better
> idea, please speak up.
> 


Something related that you might be interested in:
https://lkml.org/lkml/2012/2/5/109

(This is part of Gilad's patchset that tries to reduce cross-CPU IPI
interference.)

Regards,
Srivatsa S. Bhat


> diff --git a/fs/buffer.c b/fs/buffer.c
> index 1a30db7..56b0d2b 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1384,10 +1384,31 @@ static void invalidate_bh_lru(void *arg)
>  	}
>  	put_cpu_var(bh_lrus);
>  }
> -	
> +
> +/*
> + * Invalidate all buffers in LRUs. Since we have to signal all CPUs to
> + * invalidate their per-cpu local LRU lists this is rather expensive operation.
> + * So we optimize the case of several parallel calls to invalidate_bh_lrus()
> + * which happens from partitioning code when lots of disks appear in the
> + * system during boot.
> + */
>  void invalidate_bh_lrus(void)
>  {
> +	static DEFINE_MUTEX(bh_invalidate_mutex);
> +	static long bh_invalidate_sequence;
> +
> +	long my_bh_invalidate_sequence = bh_invalidate_sequence;
> +
> +	mutex_lock(&bh_invalidate_mutex);
> +	/* Someone did bh invalidation while we were sleeping? */
> +	if (my_bh_invalidate_sequence != bh_invalidate_sequence)
> +		goto out;
> +	bh_invalidate_sequence++;
> +	/* Inc of bh_invalidate_sequence must happen before we invalidate bhs */
> +	smp_wmb();
>  	on_each_cpu(invalidate_bh_lru, NULL, 1);
> +out:
> +	mutex_unlock(&bh_invalidate_mutex);
>  }
>  EXPORT_SYMBOL_GPL(invalidate_bh_lrus);
> 

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