linux-kernel - Re: rcu: performance regression

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Message-ID: <20110614124323.GC2264@linux.vnet.ibm.com>
Date:	Tue, 14 Jun 2011 05:43:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	"Chen, Tim C" <tim.c.chen@...el.com>,
	"Shi, Alex" <alex.shi@...el.com>
Subject: Re: rcu: performance regression

On Tue, Jun 14, 2011 at 01:26:25PM +0800, Shaohua Li wrote:
> Commit a26ac2455ffcf3(rcu: move TREE_RCU from softirq to kthread)
> introduced performance regression. In our AIM7 test, this commit caused
> about 40% regression.
> The commit runs rcu callbacks in a kthread instead of softirq. We
> observed high rate of context switch which is caused by this. Out test
> system has 64 CPUs and HZ is 1000, so we saw more than 64k context
> switch per second which is caused by the rcu thread.
> I also did trace and found when rcy thread is woken up, most time the
> thread doesn't handle any callbacks actually, it just initializes new gp
> or end one gp or similar.
> >From my understanding, the purpose to make rcu runs in kthread is to
> speed up rcu callbacks run (with help of rtmutex PI), not for end gp and
> so on, which runs pretty fast actually and doesn't need boost.
> To verify my findings, I had below debug patch applied. It still handles
> rcu callbacks in kthread if there is any pending callbacks, but other
> things are still running in softirq. this completely solved our
> regression. I thought this can still boost callbacks run. but I'm not
> expert in the area, so please help.

Hello, Shaohua,

Could you please try the following patch?  In the meantime, I will
also look over the patch that you sent.

							Thanx, Paul
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