Re: [Xen-devel] net: Detect drivers that reschedule NAPI and exhaust budget

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

On Fri, Dec 19, 2014 at 09:40:00PM -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Date: Fri, 19 Dec 2014 17:34:48 -0800
> 
> >> @@ -4620,7 +4620,11 @@ static void net_rx_action(struct softirq_action *h)
> >>                                     */
> >>                                    napi_gro_flush(n, HZ >= 1000);
> >>                            }
> >> -                          list_add_tail(&n->poll_list, &repoll);
> >> +                          /* Some drivers may have called napi_schedule
> >> +                           * prior to exhausting their budget.
> >> +                           */
> >> +                          if (!WARN_ON_ONCE(!list_empty(&n->poll_list)))
> >> +                                  list_add_tail(&n->poll_list, &repoll);
> >>                    }
> >>            }
> >>  
> > 
> > I do not think stack trace will point to the buggy driver ?
> > 
> > IMO it would be better to print a single line with the netdev name ?
> 
> Right, we are already back from the poll routine and will just end
> up seeing the call trace leading to the software interrupt.

Good point Eric.

-- >8 --
The commit d75b1ade567ffab085e8adbbdacf0092d10cd09c (net: less
interrupt masking in NAPI) required drivers to leave poll_list
empty if the entire budget is consumed.

We have already had two broken drivers so let's add a check for
this.

Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

diff --git a/net/core/dev.c b/net/core/dev.c
index f411c28..47fdc5c 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4620,7 +4620,13 @@ static void net_rx_action(struct softirq_action *h)
                                         */
                                        napi_gro_flush(n, HZ >= 1000);
                                }
-                               list_add_tail(&n->poll_list, &repoll);
+                               /* Some drivers may have called napi_schedule
+                                * prior to exhausting their budget.
+                                */
+                               if (unlikely(!list_empty(&n->poll_list)))
+                                       pr_warn("%s: Budget exhausted after 
napi rescheduled\n", n->dev ? n->dev->name : "backlog");
+                               else
+                                       list_add_tail(&n->poll_list, &repoll);
                        }
                }
 
Thanks,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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