netdev - Re: RFS issue: no HW filter for paused stream

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Date:	Mon, 19 Sep 2011 16:52:52 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Tom Herbert <therbert@...gle.com>,
	Amir Vadai <amirv@....mellanox.co.il>
Cc:	oren@...lanox.co.il, liranl@...lanox.co.il, netdev@...r.kernel.org,
	amirv@...lanox.co.il
Subject: Re: RFS issue: no HW filter for paused stream

On Mon, 2011-09-19 at 08:13 -0700, Tom Herbert wrote:
> Ben:  Once a accel RFS flow expires (because flow is idle?), how
> should it get re-instantiated if thread's CPU doesn't change?

Good question.

> Tom
> 
> On Sun, Sep 18, 2011 at 11:05 PM, Amir Vadai <amirv@....mellanox.co.il> wrote:
> > (Resending in plain text)
> >
> > Tom Hi,
> > When a stream is paused, and its rule is expired while it is paused,
> > no new rule will be configured to the HW when traffic resume.
> >
> > Scenario:
> > 1. Start iperf.
> > 2. Pause it using Ctrl-Z
> > 3. Start another iperf (to make sure first stream rule is expired)
> > 4. Stop the second stream.
> > 5. Resume first stream. Traffic is not steered to the right rx-queue.
> >
> > From looking at the code:
> > - When first stream started, RSS steered traffic to rx-queue 'x'.
> > Because iperf server was running on a different CPU, a new rule was
> > added and current-cpu was set to desired-cpu.
> > - After paused, rule was expired and removed from HW by net driver.
> > But current-cpu wasn't cleared and still is equal to desired-cpu.
> > - When stream was resumed, traffic was steered again by RSS, and
> > because current-cpu was equal to desired-cpu,  ndo_rx_flow_steer
> > wasn't called and no rule was configured to the HW.
> >
> > Why isn't current-cpu cleared when expiring a rule?

Because I wrongly assumed that rules could be independently expired by
the driver and the RPS/RFS core code.

Try this (I haven't tested it myself yet):

From: Ben Hutchings <bhutchings@...arflare.com>
Date: Mon, 19 Sep 2011 16:44:13 +0100
Subject: [PATCH net-next] RPS: When a hardware filter is expired, ensure it
 can be re-added later

Amir Vadai wrote:
> When a stream is paused, and its rule is expired while it is paused,
> no new rule will be configured to the HW when traffic resume.
[...]
> - When stream was resumed, traffic was steered again by RSS, and
> because current-cpu was equal to desired-cpu,  ndo_rx_flow_steer
> wasn't called and no rule was configured to the HW.
>
> Why isn't current-cpu cleared when expiring a rule?

When rps_may_expire_flow() matches a filter to a flow that is found to
be idle, unset the current CPU for that flow.

Reported-by: Amir Vadai <amirv@....mellanox.co.il>
---
 net/core/dev.c |   21 ++++++++++++++++-----
 1 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index b2e262e..3caf65a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2817,11 +2817,22 @@ bool rps_may_expire_flow(struct net_device *dev, u16 rxq_index,
 	if (flow_table && flow_id <= flow_table->mask) {
 		rflow = &flow_table->flows[flow_id];
 		cpu = ACCESS_ONCE(rflow->cpu);
-		if (rflow->filter == filter_id && cpu != RPS_NO_CPU &&
-		    ((int)(per_cpu(softnet_data, cpu).input_queue_head -
-			   rflow->last_qtail) <
-		     (int)(10 * flow_table->mask)))
-			expire = false;
+		if (rflow->filter == filter_id && cpu != RPS_NO_CPU) {
+			if ((int)(per_cpu(softnet_data, cpu).input_queue_head -
+				  rflow->last_qtail) <
+			    (int)(10 * flow_table->mask)) {
+				expire = false;
+			} else {
+				/* If this flow (or a flow with the
+				 * same hash value) becomes active
+				 * on the CPU as before, we want to
+				 * restore the hardware filter.  Unset
+				 * the current CPU to ensure that
+				 * set_rps_cpu() will be called then.
+				 */
+				rflow->cpu = RPS_NO_CPU;
+			}
+		}
 	}
 	rcu_read_unlock();
 	return expire;
-- 
1.7.4.4

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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