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

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Date:	Tue, 13 Nov 2007 11:05:38 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	benh@...nel.crashing.org
Cc:	paulmck@...ux.vnet.ibm.com, Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, tony@...eyournoodle.com,
	paulus@...ba.org, dino@...ibm.com, tytso@...ibm.com,
	dvhltc@...ibm.com, niv@...ibm.com, antonb@...ibm.com,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH, RFC] improved hacks to allow -rt to run kernbench on
	POWER


On Tue, 2007-11-13 at 10:10 +0100, Peter Zijlstra wrote:
> 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?

Looking at the code:

/* radix tree not lockless safe ! we use a brlock-type mecanism
 * for now, until we can use a lockless radix tree
 */
static void irq_radix_wrlock(unsigned long *flags)

The RCU radix tree stuffs have gone upstream long ago.

Anyway, it seems its the generic irq stuff that uses raw_spinlock_t and
disables IRQs, so there isn't much we can do from the ARCH level I'm
afraid :-(

Ingo, any sane ideas?

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