netdev - Re: [PATCH net-next] net: sched: use pinned timers

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Message-Id: <20140926.002710.1595423408687413602.davem@davemloft.net>
Date:	Fri, 26 Sep 2014 00:27:10 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: sched: use pinned timers

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sat, 20 Sep 2014 18:01:30 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> While using a MQ + NETEM setup, I had confirmation that the default
> timer migration ( /proc/sys/kernel/timer_migration ) is killing us.
> 
> Installing this on a receiver side of a TCP_STREAM test, (NIC has 8 TX
> queues) :
 ...
> We can see that timers get migrated into a single cpu, presumably idle
> at the time timers are set up.
> Then all qdisc dequeues run from this cpu and huge lock contention
> happens. This single cpu is stuck in softirq mode and cannot dequeue
> fast enough.
> 
>     39.24%  [kernel]          [k] _raw_spin_lock
>      2.65%  [kernel]          [k] netem_enqueue
>      1.80%  [kernel]          [k] netem_dequeue                         
>      1.63%  [kernel]          [k] copy_user_enhanced_fast_string
>      1.45%  [kernel]          [k] _raw_spin_lock_bh        
> 
> By pinning qdisc timers on the cpu running the qdisc, we respect proper
> XPS setting and remove this lock contention.
> 
>      5.84%  [kernel]          [k] netem_enqueue                      
>      4.83%  [kernel]          [k] _raw_spin_lock
>      2.92%  [kernel]          [k] copy_user_enhanced_fast_string
> 
> Current Qdiscs that benefit from this change are :
> 
> 	netem, cbq, fq, hfsc, tbf, htb.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Looks great, applied, thanks Eric.
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