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

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Date:	Sat, 25 Feb 2012 12:24:03 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Mike Galbraith <efault@....de>
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

* 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

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)?

- vatsa

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