linux-kernel - Re: [PATCH tip/core/rcu 55/55] powerpc: Work around tracing from dyntick-idle mode

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Message-ID: <20110915145307.GB31630@somewhere.redhat.com>
Date:	Thu, 15 Sep 2011 16:53:11 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.org, anton@...ba.org, paulus@...ba.org
Subject: Re: [PATCH tip/core/rcu 55/55] powerpc: Work around tracing from
 dyntick-idle mode

On Tue, Sep 13, 2011 at 05:49:53PM -0300, Benjamin Herrenschmidt wrote:
> 
> > As I understand it, cede_processor()'s call to plpar_hcall_norets()
> > results in the hypervisor being invoked, and could give up the CPU.
> > And yes, in this case, RCU needs to stop paying attention to this CPU.
> > And pseries_shared_idle_sleep() also invokes cede_proceessor().
> > 
> > Gah...  And there also appear to be some assembly-language functions
> > that can be invoked via the ppc_md.power_save() call from cpu_idle():
> > ppc6xx_idle(), power4_idle(), idle_spin(), idle_doze(), and book3e_idle().
> > There is also a power7_idle(), but it does not appear to be used anywhere.
> > 
> > Plus there are the C-language ppc44x_idle(), beat_power_save(),
> > cbe_power_save(), ps3_power_save(), and cpm_idle().
> > 
> > > > The same thing would be needed for tick_nohz_exit_idle() and
> > > > rcu_exit_nohz(): powerpc would need to invoke rcu_exit_nohz() after
> > > > gaining control from the hypervisor but before doing its first tracing,
> > > > and then it would need the idle loop to to tick_nohz_exit_idle(false).
> > > > Again, if pseries is the only powerpc architecture requiring this,
> > > > the argument to tick_nohz_exit_idle() could depend on the architecture.
> > > > 
> > > > Would this approach work?
> > > 
> > > Sounds like we really need that.
> > 
> > Sounds like an arch-dependent config symbol that is defined for the
> > pseries targets, but not for the other powerpc architectures.
> > 
> > Not clear to me what to do about power4_idle(), though.
> 
> I don't totally follow, too many things to deal with right now, but keep
> in mind that we build multiplatform kernels, so you can have powermac,
> cell, pseries, etc... all in one kernel binary (including power7 idle).
> 
> Shouldn't we instead change the plpar trace call to skip the tracing
> when not safe to do so ?

Or may be we can have a plpar_hcall_norets_notrace() for this specific case?
Lemme try something.
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