linux-kernel - Re: [RFC] IB/mad: Use IDR instead of per-port list

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Message-ID: <20180607234926.GA8260@bombadil.infradead.org>
Date:   Thu, 7 Jun 2018 16:49:26 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     hans.westgaard.ry@...cle.com, Doug Ledford <dledford@...hat.com>,
        linux-rdma@...r.kernel.org,
        Håkon Bugge <haakon.bugge@...cle.com>,
        Parav Pandit <parav@...lanox.com>,
        Jack Morgenstein <jackm@....mellanox.co.il>,
        Pravin Shedge <pravin.shedge4linux@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] IB/mad: Use IDR instead of per-port list

On Thu, Jun 07, 2018 at 04:26:46PM -0600, Jason Gunthorpe wrote:
> On Thu, Jun 07, 2018 at 12:08:32PM -0700, Matthew Wilcox wrote:
> > 
> > Hans reports a bug where the mlx4 driver uses the MSB of the agent number
> > to store slave number, meaning we can only use agent numbers up to 2^24.
> > Fix this by using an IDR to assign the agent numbers and also look up the
> > agent when a MSD packet is received.
> > 
> > I've changed the locking substantially, so this may well have a
> > performance issue.  There are a few different possibilities for fixing
> > that, including moving to an RCU-based lookup.
> 
> I do like this better than the last series..
> 
> This are is somewhat performance sensitive and it would be nice to
> avoid this global lock.

OK, I wasn't sure whether it was worth it.

> What about using a read/write spinlock instead of the IDR internal
> lock? Then all the per-port reading threads calling find_mad_agent can
> run concurrently..

It'd be better to switch to RCU ... the IDR is RCU-safe, but the
version/class/method or OUI match isn't.  Do you have any feeling on
the relative frequency of the two types of "routing"?

Actually, I think we can use the radix tree data structure for the
version/class/method too ... that's going to take a little more work.