Re: [PATCH, RFC] improved hacks to allow -rt to run kernbench on POWER

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2007-11-13 at 08:12 +1100, Benjamin Herrenschmidt wrote:
> > And here is an updated patch.  There has to be a better way than the
> > #ifdef, but I need the two local variables, and breaking the intervening
> > code out into a separate function didn't quite seem right either.
> > 
> > Thoughts?
> 
> Nothing comes to mind right now...
> 
> > This one does only one oops during boot-up, which I will start looking
> > at:
> > 
> > BUG: sleeping function called from invalid context ifconfig(994) at kernel/rtmutex.c:637
> > in_atomic():1 [00000002], irqs_disabled():1
> > Call Trace:
> > [c0000000ff49f030] [c000000000010028] .show_stack+0x6c/0x1a0 (unreliable)
> > [c0000000ff49f0d0] [c00000000004f8b4] .__might_sleep+0x11c/0x138
> > [c0000000ff49f150] [c00000000039c920] .__rt_spin_lock+0x38/0xa0
> > [c0000000ff49f1d0] [c0000000000cf8e8] .kmem_cache_alloc+0x68/0x184
> > [c0000000ff49f270] [c0000000001f7534] .radix_tree_node_alloc+0x3c/0x104
> > [c0000000ff49f300] [c0000000001f8418] .radix_tree_insert+0x19c/0x324
> > [c0000000ff49f3c0] [c00000000000b758] .irq_radix_revmap+0x140/0x178
> > [c0000000ff49f470] [c000000000044aec] .xics_startup+0x30/0x54
> > [c0000000ff49f500] [c0000000000997f4] .setup_irq+0x254/0x320
> > [c0000000ff49f5b0] [c000000000099984] .request_irq+0xc4/0x114
> > [c0000000ff49f660] [d00000000079b194] .e1000_open+0xdc/0x1b8 [e1000]
> > [c0000000ff49f6f0] [c00000000030a840] .dev_open+0x94/0x110
> > [c0000000ff49f790] [c00000000030a69c] .dev_change_flags+0x110/0x220
> > [c0000000ff49f830] [c00000000036a0a8] .devinet_ioctl+0x2cc/0x764
> > [c0000000ff49f930] [c00000000036a6a8] .inet_ioctl+0xe8/0x138
> > [c0000000ff49f9b0] [c0000000002f9acc] .sock_ioctl+0x2c8/0x314
> > [c0000000ff49fa50] [c0000000000e6dec] .do_ioctl+0x5c/0xf0
> > [c0000000ff49faf0] [c0000000000e731c] .vfs_ioctl+0x49c/0x4d4
> > [c0000000ff49fba0] [c0000000000e73ec] .sys_ioctl+0x98/0xe0
> > [c0000000ff49fc50] [c000000000117944] .dev_ifsioc+0x1e0/0x46c
> > [c0000000ff49fd40] [c00000000011e1d4] .compat_sys_ioctl+0x40c/0x4a0
> > [c0000000ff49fe30] [c00000000000852c] syscall_exit+0x0/0x40
> 
> The radix tree is used by the powerpc IRQ subsystem with some hand-made
> locking that involves per-cpu variables among others, you may want to
> have a look at arch/powerpc/kernel/irq.c ... It should all be GFP_ATOMIC
> though, but if -rt can't cope with even GFP_ATOMIC when preempt is off,
> then we have a deeper problem (the allocation of the page for RCU in
> freeing page tables is another one that will GFP_ATOMIC in
> non-preemptible context afaik).

Correct, -rt can't allocate -anything- when preemption if off. That is
the cost for having the allocators itself preemptable.

Even radix_tree_preload() will not work as its functionality was based
on preempt disable to limit access to a global (per cpu) object reserve.
But maybe something similar could be done with a local reserve by using
struct radix_tree_context to pass it along.

I'll see if I can come up with anything like that, that is, if that
would suffice?

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