Re: [PATCH] set_pgdat_percpu_threshold() don't use for_each_online_cpu

[Christoph Lameter <cl@linux.com>]

On Tue, 16 Nov 2010, Andrew Morton wrote:

> > Following patch use (2) beucase removing get_online_cpus() makes good
> > side effect. It reduce potentially cpu-hotplug vs memory-shortage deadlock
> > risk.
>
> Well.  Being able to run for_each_online_cpu() is a pretty low-level
> and fundamental thing.  It's something we're likely to want to do more
> and more of as time passes.  It seems a bad thing to tell ourselves
> that we cannot use it in reclaim context.  That blots out large chunks
> of filesystem and IO-layer code as well!

The online map can change if no locks were taken. Thus it
becomes something difficult to do in some code paths and overhead
increases significantly.

> >  		threshold = (*calculate_pressure)(zone);
> > -		for_each_online_cpu(cpu)
> > +		for_each_possible_cpu(cpu)
> >  			per_cpu_ptr(zone->pageset, cpu)->stat_threshold
> >  							= threshold;
> >  	}
> > -	put_online_cpus();
> >  }
>
> That's a pretty sad change IMO, especially of num_possible_cpus is much
> larger than num_online_cpus.

num_possible_cpus should only be higher if the arch code has detected
that the system has the ability to physically online and offline cpus.
I have never actually seen such a system. Heard rumors from Fujitsu that
they have something.

Maybe the virtualization people also need this? Otherwise cpu
online/offline is useful mainly to debug the cpu offline/online handling
in various subsystems which is unsurprisingly often buggy given the rarity
of encountering such hardware.

> What do we need to do to make get_online_cpus() safe to use in reclaim
> context?  (And in kswapd context, if that's really equivalent to
> "reclaim context").

I think its not worth the effort.

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