netdev - Re: [v2 PATCH 7/10] rhashtable: Disable automatic shrinking

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Message-ID: <20150323094321.GE16023@casper.infradead.org>
Date:	Mon, 23 Mar 2015 09:43:21 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Patrick McHardy <kaber@...sh.net>,
	Josh Triplett <josh@...htriplett.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org
Subject: Re: [v2 PATCH 7/10] rhashtable: Disable automatic shrinking

On 03/23/15 at 08:29pm, Herbert Xu wrote:
> On Mon, Mar 23, 2015 at 08:37:12AM +0000, Thomas Graf wrote:
> >
> > I think rhashtable_shrink() should fetch ht->tbl in an RCU section to
> > cheaply get the current table size and only do the allocation and take
> > the lock if the table size warrants for shrinking.
> 
> Well you should never invoke rhashtable_shrink unless you actually
> wanted to shrink.  So this is something that you should have checked
> before rhashtable_shrink is called.

How? The calculation of the table size is embedded in
rhashtable_shrink(). Should every user have a copy of that
calculation algorithm?

Why not just:

	unlikely(ht->p.shrink && rht_shrink_below_30(..))

If you really care about that additional conditional we
can also add:

static inline int rhashtable_remove_and_shrink()
{
        int err;

        rcu_read_lock();

	tbl = rht_dereference_rcu(ht->tbl, ht);

        err = rhashtable_remove_fast();
        if (unlikely(!err && rht_shrink_below_30(ht, tbl)))
                schedule_work(&ht->run_work);

	rcu_read_unlock();

	return err;
}

I just think it's wrong to rip out all the shrinking logic and
require every single user to re-add its own copy.
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