Re: [PATCH v5 7/8] mm: Only IPI CPUs to drain local pages if theyexist

[Paul E. McKenney]

On Thu, Jan 05, 2012 at 04:35:29PM +0000, Russell King - ARM Linux wrote:
> On Thu, Jan 05, 2012 at 04:17:39PM +0000, Mel Gorman wrote:
> > Link please?
> 
> Forwarded, as its still in my mailbox.
> 
> > I'm including a patch below under development that is
> > intended to only cope with the page allocator case under heavy memory
> > pressure. Currently it does not pass testing because eventually RCU
> > gets stalled with the following trace
> > 
> > [ 1817.176001]  [<ffffffff810214d7>] arch_trigger_all_cpu_backtrace+0x87/0xa0
> > [ 1817.176001]  [<ffffffff810c4779>] __rcu_pending+0x149/0x260
> > [ 1817.176001]  [<ffffffff810c48ef>] rcu_check_callbacks+0x5f/0x110
> > [ 1817.176001]  [<ffffffff81068d7f>] update_process_times+0x3f/0x80
> > [ 1817.176001]  [<ffffffff8108c4eb>] tick_sched_timer+0x5b/0xc0
> > [ 1817.176001]  [<ffffffff8107f28e>] __run_hrtimer+0xbe/0x1a0
> > [ 1817.176001]  [<ffffffff8107f581>] hrtimer_interrupt+0xc1/0x1e0
> > [ 1817.176001]  [<ffffffff81020ef3>] smp_apic_timer_interrupt+0x63/0xa0
> > [ 1817.176001]  [<ffffffff81449073>] apic_timer_interrupt+0x13/0x20
> > [ 1817.176001]  [<ffffffff8116c135>] vfsmount_lock_local_lock+0x25/0x30
> > [ 1817.176001]  [<ffffffff8115c855>] path_init+0x2d5/0x370
> > [ 1817.176001]  [<ffffffff8115eecd>] path_lookupat+0x2d/0x620
> > [ 1817.176001]  [<ffffffff8115f4ef>] do_path_lookup+0x2f/0xd0
> > [ 1817.176001]  [<ffffffff811602af>] user_path_at_empty+0x9f/0xd0
> > [ 1817.176001]  [<ffffffff81154e7b>] vfs_fstatat+0x4b/0x90
> > [ 1817.176001]  [<ffffffff81154f4f>] sys_newlstat+0x1f/0x50
> > [ 1817.176001]  [<ffffffff81448692>] system_call_fastpath+0x16/0x1b
> > 
> > It might be a separate bug, don't know for sure.

Do you get multiple RCU CPU stall-warning messages?  If so, it can
be helpful to look at how the stack frame changes over time.  These
stalls are normally caused by a loop in the kernel with preemption
disabled, though other scenarios can also cause them.

I am assuming that the CPU is reporting a stall on itself in this case.
If not, then it is necessary to look at the stack of the CPU that the
stall is being reported for.

							Thanx, Paul

> I'm not going to even pretend to understand what the above backtrace
> means: it doesn't look like what I'd expect from the problem which
> PeterZ's patch is supposed to address.  It certainly doesn't do anything
> to address the cpu-going-offline problem you seem to have found.
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