Re: RFC: issues concerning the next NAPI interface

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David Miller wrote:
From: James Chapman <[email protected]>
Date: Mon, 27 Aug 2007 16:51:29 +0100

To implement this, there's no need for timers, hrtimers or generic NAPI support that others have suggested. A driver's poll() would set an internal flag and record the current jiffies value when finding workdone=0 rather than doing an immediate napi_complete(). Early in poll() it would test this flag and if set, do a low-cost test to see if it had any work to do. If no work, it would check the saved jiffies value and do the napi_complete() only if no work has been done for a configurable number of jiffies. This keeps interrupts disabled longer at the expense of many more calls to poll() where no work is done. So critical to this scheme is modifying the driver's poll() to fastpath the case of having no work to do while waiting for its local jiffy count to expire.
Here's an untested patch for tg3 that illustrates the idea.
It's only going to work with hrtimers, these interfaces can
process at least 100,000 per jiffies tick.
I don't understand where hrtimers or interface speed comes in. If the
CPU is fast enough to call poll() 100,000 times per jiffies tick, it
means 100,000 wasted poll() calls while the netdev migrates from active
to poll-off state. Hence the need to fastpath the "no work" case in the
netdev's poll(). These extra poll() calls are tolerable if it avoids
NAPI thrashing between poll-on and poll-off states for certain packet rates.
And the hrtimer granularity is going to need to be significantly low,
and futhermore you're adding a guaranteed extra interrupt (for the
hrtimer firing) in these cases where we're exactly trying to avoid is
more interrupts.

If you can make it work, fine, but it's going to need to be at a
minimum disabled when the hrtimer granularity is not sufficient.

But there are huger fish to fry for you I think.  Talk to your
platform maintainers and ask for an interface for obtaining
a flat static distribution of interrupts to cpus in order to
support multiqueue NAPI better.

In your previous postings you made arguments saying that the
automatic placement of interrupts to cpus made everything
bunch of to a single cpu and you wanted to propagate the
NAPI work to other cpu's software interrupts from there.
I don't recall saying anything in previous posts about this. Are you
confusing my posts with Jan-Bernd's? Jan-Bernd has been talking about
using hrtimers to _reschedule_ NAPI. My posts are suggesting an
alternative mechanism that keeps NAPI active (with interrupts disabled)
for a jiffy or two after it would otherwise have gone idle in order to
avoid too many interrupts when the packet rate is such that NAPI
thrashes between poll-on and poll-off.
That logic is bogus, because it merely proves that the hardware
interrupt distribution is broken.  If it's a bad cpu to run
software interrupts on, it's also a bad cpu to run hardware
interrupts on.
James Chapman
Katalix Systems Ltd
Catalysts for your Embedded Linux software development

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