linux-kernel - Re: sched: Avoid SMT siblings in select_idle

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Date:	Sat, 25 Feb 2012 09:30:55 +0100
From:	Mike Galbraith <efault@....de>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>
Subject: Re: sched: Avoid SMT siblings in select_idle_sibling() if possible

On Sat, 2012-02-25 at 12:24 +0530, Srivatsa Vaddagiri wrote: 
> * Mike Galbraith <efault@....de> [2012-02-23 12:21:04]:
> 
> > Unpinned netperf TCP_RR and/or tbench pairs?  Anything that's wakeup
> > heavy should tell the tail.
> 
> Here are some tbench numbers:
> 
> Machine : 2 Intel Xeon X5650 (Westmere) CPUs (6 core/package)
> Kernel  : tip (HEAD at ebe97fa)
> dbench  : v4.0
> 
> One tbench server/client pair was run on same host 5 times (with
> fs-cache being purged each time) and avg of 5 run for various cases
> noted below:
> 
> Case A : HT enabled (24 logical CPUs)
> 
> Thr'put	 : 168.166 MB/s (SD_SHARE_PKG_RESOURCES + !SD_BALANCE_WAKE)
> Thr'put	 : 169.564 MB/s (SD_SHARE_PKG_RESOURCES + SD_BALANCE_WAKE at mc/smt)
> Thr'put	 : 173.151 MB/s (!SD_SHARE_PKG_RESOURCES + !SD_BALANCE_WAKE)
> 
> Case B : HT disabled (12 logical CPUs)
> 
> Thr'put	 : 167.977 MB/s (SD_SHARE_PKG_RESOURCES + !SD_BALANCE_WAKE)
> Thr'put	 : 167.891 MB/s (SD_SHARE_PKG_RESOURCES + SD_BALANCE_WAKE at mc)
> Thr'put	 : 173.801 MB/s (!SD_SHARE_PKG_RESOURCES + !SD_BALANCE_WAKE)
> 
> Observations:
> 
> a. ~3% improvement seen with SD_SHARE_PKG_RESOURCES disabled, which I guess 
>   reflects the cost of waking to a cold L2 cache.
> 
> b. No degradation seen with SD_BALANCE_WAKE enabled at mc/smt domains

I haven't done a lot of testing, but yeah, the little I have doesn't
show SD_BALANCE_WAKE making much difference on single socket boxen.

> IMO we need to detect tbench type paired wakeups as synchronous case, in
> which case blindly wakeup the task to cur_cpu (as cost of L2 cache miss
> could outweight the cost of any reduced scheduling latencies).
> 
> IOW select_task_rq_fair() needs to be given better hint as to whether L2
> cache has been made warm by someone (interrupt handler or a producer
> task), in which case (consumer) task needs to be woken in the same L2 cache
> domain (i.e on cur_cpu itself)?

My less rotund config shows the L2 penalty decidedly more prominently.
We used to have avg_overlap as a synchronous wakeup hint, but it was
broken by preemption and whatnot, got the axe to recover some cycles.  A
reliable and dirt cheap replacement would be a good thing to have.

TCP_RR and tbench are far way away from the overlap breakeven point on
E5620, whereas with Q6600s shared L2, you can start converting overlap
into throughput almost immediately. 

2.4 GHz E5620
Throughput 248.994 MB/sec 1 procs  SD_SHARE_PKG_RESOURCES
Throughput 379.488 MB/sec 1 procs  !SD_SHARE_PKG_RESOURCES

2.4 GHz Q6600
Throughput 299.049 MB/sec 1 procs  SD_SHARE_PKG_RESOURCES
Throughput 300.018 MB/sec 1 procs  !SD_SHARE_PKG_RESOURCES

-Mike

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