Date:	Sun, 17 Apr 2011 13:20:10 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>, netdev@...r.kernel.org,
	davem@...emloft.net
Subject: Re: net: Automatic IRQ siloing for network devices

On Sat, Apr 16, 2011 at 09:17:04AM -0700, Stephen Hemminger wrote:
> On Fri, 15 Apr 2011 21:59:38 -0400
> Neil Horman <nhorman@...driver.com> wrote:
> 
> > On Fri, Apr 15, 2011 at 11:54:29PM +0100, Ben Hutchings wrote:
> > > On Fri, 2011-04-15 at 16:17 -0400, Neil Horman wrote:
> > > > Automatic IRQ siloing for network devices
> > > > 
> > > > At last years netconf:
> > > > http://vger.kernel.org/netconf2010.html
> > > > 
> > > > Tom Herbert gave a talk in which he outlined some of the things we can do to
> > > > improve scalability and througput in our network stack
> > > > 
> > > > One of the big items on the slides was the notion of siloing irqs, which is the
> > > > practice of setting irq affinity to a cpu or cpu set that was 'close' to the
> > > > process that would be consuming data.  The idea was to ensure that a hard irq
> > > > for a nic (and its subsequent softirq) would execute on the same cpu as the
> > > > process consuming the data, increasing cache hit rates and speeding up overall
> > > > throughput.
> > > > 
> > > > I had taken an idea away from that talk, and have finally gotten around to
> > > > implementing it.  One of the problems with the above approach is that its all
> > > > quite manual.  I.e. to properly enact this siloiong, you have to do a few things
> > > > by hand:
> > > > 
> > > > 1) decide which process is the heaviest user of a given rx queue 
> > > > 2) restrict the cpus which that task will run on
> > > > 3) identify the irq which the rx queue in (1) maps to
> > > > 4) manually set the affinity for the irq in (3) to cpus which match the cpus in
> > > > (2)
> > > [...]
> > > 
> > > This presumably works well with small numbers of flows and/or large
> > > numbers of queues.  You could scale it up somewhat by manipulating the
> > > device's flow hash indirection table, but that usually only has 128
> > > entries.  (Changing the indirection table is currently quite expensive,
> > > though that could be changed.)
> > > 
> > > I see RFS and accelerated RFS as the only reasonable way to scale to
> > > large numbers of flows.  And as part of accelerated RFS, I already did
> > > the work for mapping CPUs to IRQs (note, not the other way round).  If
> > > IRQ affinity keeps changing then it will significantly undermine the
> > > usefulness of hardware flow steering.
> > > 
> > > Now I'm not saying that your approach is useless.  There is more
> > > hardware out there with flow hashing than with flow steering, and there
> > > are presumably many systems with small numbers of active flows.  But I
> > > think we need to avoid having two features that conflict and a
> > > requirement for administrators to make a careful selection between them.
> > > 
> > > Ben.
> > > 
> > I hear what your saying and I agree, theres no point in having features work
> > against each other.  That said, I'm not sure I agree that these features have to
> > work against one another, nor does a sysadmin need to make a choice between the
> > two.  Note the third patch in this series.  Making this work requires that
> > network drivers wanting to participate in this affinity algorithm opt in by
> > using the request_net_irq macro to attach the interrupt to the rfs affinity code
> > that I added.  Theres no reason that a driver which supports hardware that still
> > uses flow steering can't opt out of this algorithm, and as a result irqbalance
> > will still treat those interrupts as it normally does.  And for those drivers
> > which do opt in, irqbalance can take care of affinity assignment, using the
> > provided hint.  No need for sysadmin intervention.
> > 
> > I'm sure there can be improvements made to this code, but I think theres less
> > conflict between the work you've done and this code than there appears to be at
> > first blush.
> > 
> 
> My gut feeling is that:
>   * kernel should default to a simple static sane irq policy without user
>     space.  This is especially true for multi-queue devices where the default
>     puts all IRQ's on one cpu.
> 
Thats not how it currently works, AFAICS.  The default kernel policy is
currently that cpu affinity for any newly requested irq is all cpus.  Any
restriction beyond that is the purview and doing of userspace (irqbalance or
manual affinity setting).

>   * irqbalance should do a one-shot rearrangement at boot up. It should rearrange
>     when new IRQ's are requested. The kernel should have capablity to notify
>     userspace (uevent?) when IRQ's are added or removed.
> 
I can see that, and it would be easy to implement.  That said, I'm not sure what
criteria should be used when doing said re-arrangement.  Currently irqbalance
uses interrupt counts and names to determine how interrupts should be placed.
That is of course a hack, but its done because its currently the best
information available to user space.  Thats what this patch series was hoping to
address.  By exporting RFS flow data we give the opportunity to irqbalance to do
some modicum of better irq placement.

>   * Let scheduler make decisions about migrating processes (rather than let irqbalance
>     migrate IRQ's).
> 
I can certainly get behind this idea, I've been having trouble however comming
up with a good algorithm that lets the scheduler make a rational decision about
which cpu to run a process on.  I.e. how to do you weigh moving a process to a
cpu thats more local to the rx queue its reciving data on against the fact that
its also sharing a memory segment with another process on its current cpu.  I'd
like to be able to normalize these comparisons, but I'm not at all sure (yet)
how to do so.

>   * irqbalance should not do the hacks it does to try and guess at network traffic.
> 
Well, I can certainly agree with that, but I'm not sure what that looks like.

I could envision something like:

1) Use irqbalance to do a one time placement of interrupts, keeping a simple
(possibly sub-optimal) policy, perhaps something like new irqs get assigned to
the least loaded cpu within the numa node of the device the irq is originating
from.

2) Add a udev event on the addition of new interrupts, to rerun irqbalance

3) Add some exported information to identify processes that are high users of
network traffic, and correlate that usage to a rxq/irq that produces that
information (possibly some per-task proc file)

4) Create/expand an additional user space daemon to monitor the highest users of
network traffic on various rxq/irqs (as identified in (3)) and restrict those
processes execution to those cpus which are on the same L2 cache as the irq
itself.  The cpuset cgroup could be usefull in doing this perhaps.

Actually, as I read back to myself, that acutally sounds kind of good to me.  It
keeps all the policy for this in user space, and minimizes what we have to add
to the kernel to make it happen (some process information in /proc and another
udev event).  I'd like to get some feedback before I start implementing this,
but I think this could be done.  What do you think?

Thanks & Regards
Neil

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