netdev - Re: [PATCH net] tcp/dccp: do not block bh too long in inet_twdr_twkill

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Date:	Sun, 12 Apr 2015 18:52:37 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net] tcp/dccp: do not block bh too long in
 inet_twdr_twkill_work()

On Sun, 2015-04-12 at 21:04 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Fri, 10 Apr 2015 04:19:07 -0700
> 
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > I have seen inet_twdr_twkill_work() blocking softirq for
> > periods up to 1.5 seconds, depending on number of timewait sockets.
> > 
> > This is an unacceptable source of latency.
> > 
> > Note that inet_twdr_do_twkill_work() releases death_lock spinlock
> > for every tw handled, but does not take care of bh enabling.
> > 
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> 
> I think it makes sense to use local_softirq_pending() here rather
> than flip the lock unconditionally.

I thought about that, but this meant having to test 2 conditions.

Not sure we need this patch anyway, if we merge the 'tcp/dccp: get rid
of central timewait timer' in 4.1 

Its not like it is a new bug ...

diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
index 6d592f8555fb..942b582d044e 100644
--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c
@@ -303,9 +303,10 @@ void inet_twdr_twkill_work(struct work_struct *work)
 				continue;
 
 			while (inet_twdr_do_twkill_work(twdr, i) != 0) {
-				if (need_resched()) {
+				if (local_softirq_pending() ||
+				    need_resched()) {
 					spin_unlock_bh(&twdr->death_lock);
-					schedule();
+					cond_resched();
 					spin_lock_bh(&twdr->death_lock);
 				}
 			}




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