Re: [PATCH, RFC] RCU : OOM avoidance and lower latency

From: Eric Dumazet <dada1@cosmosbay.com>
To: "David S. Miller" <davem@davemloft.net>
Cc: ak@suse.de, paulmck@us.ibm.com, alan@lxorguk.ukuu.org.uk,
	torvalds@osdl.org, linux-kernel@vger.kernel.org,
	dipankar@in.ibm.com, manfred@colorfullife.com,
	netdev@vger.kernel.org
Subject: Re: [PATCH, RFC] RCU : OOM avoidance and lower latency
Date: Sat, 07 Jan 2006 09:30:27 +0100	[thread overview]
Message-ID: <43BF7C23.5070805@cosmosbay.com> (raw)
In-Reply-To: <20060106.161721.124249301.davem@davemloft.net>

David S. Miller a écrit :
> 
> Eric, how important do you honestly think the per-hashchain spinlocks
> are?  That's the big barrier from making rt_secret_rebuild() a simple
> rehash instead of flushing the whole table as it does now.
> 

No problem for me in going to a single spinlock.
I did the hashed spinlock patch in order to reduce the size of the route hash 
table and not hurting big NUMA machines. If you think a single spinlock is OK, 
that's even better !

> The lock is only grabbed for updates, and the access to these locks is
> random and as such probably non-local when taken anyways.  Back before
> we used RCU for reads, this array-of-spinlock thing made a lot more
> sense.
> 
> I mean something like this patch:
> 

> +static DEFINE_SPINLOCK(rt_hash_lock);

Just one point : This should be cache_line aligned, and use one full cache 
line to avoid false sharing at least. (If a cpu takes the lock, no need to 
invalidate *rt_hash_table for all other cpus)

Eric