CONFIG_PREEMPT_RCU in next/mmotm

[Hugh Dickins]

Hi Paul,

Is CONFIG_PREEMPT_RCU=y expected to be working in -next (or mmotm)?

I ask because it appears to break down on PowerPC G5 when I try two
"make -j20" kernel builds in parallel.  The "filp" slab which usually
contains a couple of thousand objects or so, jumps up to a couple of
hundred thousand before the builds complete, and continues rising
from then on - I think that's a sign of RCU in disgrace?
And rebooting hangs thereafter.

And I notice that include/linux/rcupreempt.h currently says:
static inline void synchronize_rcu_expedited(void)
{
	synchronize_rcu();  /* Placeholder for new rcupreempt implementation. */
}
which gives an impression of work in progress?

CONFIG_PREEMPT_TREE=y seems okay on PowerPC; and when I briefly tried
CONFIG_PREEMPT_RCU=y on x86_64, it didn't show above symptoms there.

I did try bisecting yesterday's linux-next git, but that led me to

commit 8ca17c6082feee5841a7b0e91d00e18c3f85f063
Merge: dafcc6e... 7256cf0...
Author: Ingo Molnar <mingo@xxxxxxx>
Date:   Mon Aug 3 15:50:37 2009 +0200

    Merge branch 'core/rcu' into auto-sched-next

rather than to any particular patch of yours which that merges:
which seemed odd, but I'm not accustomed to bisecting next.

Another odd thing is that mmotm .DATE=2009-07-30-05-01, the last
I tried before Thursday's, showed no such symptoms: yet appears to
contain all the larger RCU changes, just lacking some more recent
on/offline race fixes.  I didn't notice any likely difference
between those mmotm trees down in arch/powerpc either.

My guess is that there's some other issue which is triggering
the RCU disgrace, but that is just a guess.

Config attached.  Any suggestions?

Thanks,
Hugh
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