Re: [PATCH] improved locking performance in rt_run_flush()

[David Miller]

From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Sun, 20 May 2007 15:11:48 +1000

> David Miller <davem@xxxxxxxxxxxxx> wrote:
> > From: Dave Johnson <djohnson+linux-kernel@xxxxxxxxxxxxxxxxxxxxxx>
> >> 
> >> The below patch changes rt_run_flush() to only take each spinlock
> >> protecting the rt_hash_table once instead of taking a spinlock for
> >> every hash table bucket (and ending up taking the same small set 
> >> of locks over and over).
> 
> ...
> 
> > I'm not ignoring it I'm just trying to brainstorm whether there
> > is a better way to resolve this inefficiency. :-)
> 
> The main problem I see with this is having to walk and free each
> chain with the lock held.  We could avoid this if we had a pointer
> in struct rtable to chain them up for freeing later.
> 
> I just checked and struct rtable is 236 bytes long on 32-bit but
> the slab cache pads it to 256 bytes so we've got some free space.
> I suspect 64-bit should be similar.

SLUB I believe packs more aggressively and won't pad things out like
that.  Therefore adding a member to rtable is much less attractive.

I've been considering various alternative ways to deal with this.

For 2.6.22 and -stable's sake we could allocate an array of pointers
of size N where N is the number of rtable hash slots per spinlock.
A big lock wraps around rt_run_flush() to protect these slots, and
then the loop is:

	grap_lock();
	for_each_hash_chain_for_lock(i) {
		rth = rt_hash_table[i].chain;
		if (rth) {
			rt_hash_table[i].chain = NULL;
			flush_chain[i % N] = rt;
		}
	}
	drop_lock();

	for (i = 0; i < N; i++) {
		struct rtable *rth = flush_chain[i];
		flush_chain[i] = NULL;
		while (rth) {
			struct rtable *next = rth->u.dst.rt_next;
			rt_free(rth);
			rth = next;
		}
	}

Holding a lock across the entire hash plucking has it's not nice
properties, but it's better than taking the same lock N times in
a row.

In the longer term, if I resurrect my dynamically sized rtable hash
patches (which I do intend to do), that code protected a lot of this
stuff with a seqlock and it might be possible to use that seqlock
solely to flush the lists in rt_run_flush().

Any better ideas?
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