linux-kernel - Re: Make yield_task

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Date:	Thu, 21 Feb 2008 15:01:43 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Subject: Re: Make yield_task_fair more efficient

Ingo Molnar wrote:
> * Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
>> If you insist that sched_yield() is bad, I might agree, but how does 
>> my patch make things worse. [...]
> 
> it puts new instructions into the hotpath.
> 
>> [...] In my benchmarks, it has helped the sched_yield case, why is 
>> that bad? [...]
> 
> I had the same cache for the rightmost task in earlier CFS (it's a 
> really obvious thing) but removed it. It wasnt a bad idea, but it hurt 
> the fastpath hence i removed it. Algorithms and implementations are a 
> constant balancing act.

This is more convincing, was the code ever in git? How did you measure the
overhead? What are your plans for reports with regressions where
kernel.compat_sched_yield is set to 1?

I have an alternate approach in mind (that I need to find time for),
threaded-rbtrees. Walking the tree is really efficient, specially finding
successor of a node.


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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