Re: [PATCH tip/core/rcu 55/55] powerpc: Work around tracing fromdyntick-idle mode

[Benjamin Herrenschmidt]

> 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 ?

Cheers,
Ben.


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