Re: [PATCH net] net_sched: avoid too many hrtimer_start() calls

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Mon, 23 May 2016 14:24:56 -0700

> From: Eric Dumazet <edumazet@google.com>
> 
> I found a serious performance bug in packet schedulers using hrtimers.
> 
> sch_htb and sch_fq are definitely impacted by this problem.
> 
> We constantly rearm high resolution timers if some packets are throttled
> in one (or more) class, and other packets are flying through qdisc on
> another (non throttled) class.
> 
> hrtimer_start() does not have the mod_timer() trick of doing nothing if
> expires value does not change :
> 
> 	if (timer_pending(timer) &&
>             timer->expires == expires)
>                 return 1;
> 
> This issue is particularly visible when multiple cpus can queue/dequeue
> packets on the same qdisc, as hrtimer code has to lock a remote base.
> 
> I used following fix :
> 
> 1) Change htb to use qdisc_watchdog_schedule_ns() instead of open-coding
> it.
> 
> 2) Cache watchdog prior expiration. hrtimer might provide this, but I
> prefer to not rely on some hrtimer internal.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>

This looks fine, applied, thanks Eric.