Question about locks in qdisc_restart

Question about locks in qdisc_restart

[Tom Herbert]

In qdisc_restart the qdisc lock is released before taking the
netif_tx_lock, and only acquired again after unlocking the
netif_tx_lock.  There's a comment with the function that
"qdisc_lock(q) and netif_tx_lock are mutually exclusive, if one is
grabbed, another must be free."  Can anyone tell me the motivation for
this restriction?  We are seeing some performance improvements by
holding the lock through qdisc_restart, and I'm not sure why this
would be bad to do.

Thanks,
Tom

Re: Question about locks in qdisc_restart

[Eric Dumazet]

Tom Herbert a écrit :
> In qdisc_restart the qdisc lock is released before taking the
> netif_tx_lock, and only acquired again after unlocking the
> netif_tx_lock.  There's a comment with the function that
> "qdisc_lock(q) and netif_tx_lock are mutually exclusive, if one is
> grabbed, another must be free."  Can anyone tell me the motivation for
> this restriction?  We are seeing some performance improvements by
> holding the lock through qdisc_restart, and I'm not sure why this
> would be bad to do.
> 

Well, motivation is to let other users (cpus) have a chance getting the lock :)

Do you see performance improvements too if using spin_is_contended()
to break the __qdisc_run loop ?

if (need_resched() || jiffies != start_time || spin_is_contended(qdisc_lock(q))

(And removing the unlock/lock in qdisc_restart() as you did)