linux-kernel - Re: [RFC PATCH v3 0/5] powerpc: Support context tracking for Power pSeries

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Message-ID: <20130529212907.GC17419@somewhere>
Date:	Wed, 29 May 2013 23:29:09 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	paulmck@...ux.vnet.ibm.com, paulus@...ba.org,
	michael@...erman.id.au, Anton Blanchard <anton@....ibm.com>
Subject: Re: [RFC PATCH v3 0/5] powerpc: Support context tracking for Power
 pSeries

On Mon, May 13, 2013 at 04:03:13PM +0800, Li Zhong wrote:
> On Mon, 2013-05-13 at 15:51 +1000, Benjamin Herrenschmidt wrote:
> > On Mon, 2013-05-13 at 13:21 +0800, Li Zhong wrote:
> > > These patches try to support context tracking for Power arch, beginning with
> > > 64-bit pSeries. The codes are ported from that of the x86_64, and in each
> > > patch, I listed the corresponding patch for x86.
> > 
> > So that's yet another pile of bloat on all syscall entry/exit and
> > exception entry/exit. What is it used for ? (I haven't followed on
> > x86_64 side).
> 
> To my understanding, it is used to enable RCU user extended quiescent
> state, so RCU on that cpu doesn't need scheduler ticks. And together
> with some other code(already in 3.10), we are able to remove the ticks
> in some cases (e.g. only 1 task running on the cpu, with some other
> limitations).
> 
> Maybe Paul, or Frederic could give some better descriptions. 

That's pretty much it. It helps RCU and cputime accounting infrastructure
to know when we enter/exit userspace. This way we can:

* Consider the CPU as idle from an RCU point of view when we run in userspace,
so RCU won't need the tick to stay alive.

* Account cputime (utime/stime/...) without using the tick. Ok powerpc already
has CONFIG_VIRT_CPU_ACCOUNTING_NATIVE but making it working with full dynticks
would require some tweaks.

All in one this is to support full dynticks.

Thanks.
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