Re: [BUGFIX] memcg CPU hotplug lockdep warning fix

[Balbir Singh]

* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> [2010-10-08 11:41:23]:

> On Fri, 8 Oct 2010 23:19:58 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > 
> > memcg has lockdep warnings (sleep inside rcu lock)
> > 
> > From: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> > 
> > Recent move to get_online_cpus() ends up calling get_online_cpus() from
> > mem_cgroup_read_stat(). However mem_cgroup_read_stat() is called under rcu
> > lock. get_online_cpus() can sleep. The dirty limit patches expose
> > this BUG more readily due to their usage of mem_cgroup_page_stat()
> > 
> > This patch address this issue as identified by lockdep and moves the
> > hotplug protection to a higher layer. This might increase the time
> > required to hotplug, but not by much.
> > 
> > Warning messages
> > 
> > BUG: sleeping function called from invalid context at kernel/cpu.c:62
> > in_atomic(): 0, irqs_disabled(): 0, pid: 6325, name: pagetest
> > 2 locks held by pagetest/6325:
> > #0:  (&mm->mmap_sem){......}, at: [<ffffffff815e9503>]
> > do_page_fault+0x27d/0x4a0
> > #1:  (rcu_read_lock){......}, at: [<ffffffff811124a1>]
> > mem_cgroup_page_stat+0x0/0x23f
> > Pid: 6325, comm: pagetest Not tainted 2.6.36-rc5-mm1+ #201
> > Call Trace:
> > [<ffffffff81041224>] __might_sleep+0x12d/0x131
> > [<ffffffff8104f4af>] get_online_cpus+0x1c/0x51
> > [<ffffffff8110eedb>] mem_cgroup_read_stat+0x27/0xa3
> > [<ffffffff811125d2>] mem_cgroup_page_stat+0x131/0x23f
> > [<ffffffff811124a1>] ? mem_cgroup_page_stat+0x0/0x23f
> > [<ffffffff810d57c3>] global_dirty_limits+0x42/0xf8
> > [<ffffffff810d58b3>] throttle_vm_writeout+0x3a/0xb4
> > [<ffffffff810dc2f8>] shrink_zone+0x3e6/0x3f8  
> > [<ffffffff81074a35>] ? ktime_get_ts+0xb2/0xbf 
> > [<ffffffff810dd1aa>] do_try_to_free_pages+0x106/0x478
> > [<ffffffff810dd601>] try_to_free_mem_cgroup_pages+0xe5/0x14c
> > [<ffffffff8110f947>] mem_cgroup_hierarchical_reclaim+0x314/0x3a2
> > [<ffffffff81111b31>] __mem_cgroup_try_charge+0x29b/0x593
> > [<ffffffff8111194a>] ? __mem_cgroup_try_charge+0xb4/0x593
> > [<ffffffff81071258>] ? local_clock+0x40/0x59  
> > [<ffffffff81009015>] ? sched_clock+0x9/0xd
> > [<ffffffff810710d5>] ? sched_clock_local+0x1c/0x82
> > [<ffffffff8111398a>] mem_cgroup_charge_common+0x4b/0x76
> > [<ffffffff81141469>] ? bio_add_page+0x36/0x38 
> > [<ffffffff81113ba9>] mem_cgroup_cache_charge+0x1f4/0x214
> > [<ffffffff810cd195>] add_to_page_cache_locked+0x4a/0x148
> > ....
> > 
> > 
> > Signed-off-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> > ---
> > 
> >  mm/memcontrol.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 116fecd..f4c5665 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -578,7 +578,6 @@ static s64 mem_cgroup_read_stat(struct mem_cgroup *mem,
> >  	int cpu;
> >  	s64 val = 0;
> >  
> > -	get_online_cpus();
> >  	for_each_online_cpu(cpu)
> >  		val += per_cpu(mem->stat->count[idx], cpu);
> >  #ifdef CONFIG_HOTPLUG_CPU
> > @@ -586,7 +585,6 @@ static s64 mem_cgroup_read_stat(struct mem_cgroup *mem,
> >  	val += mem->nocpu_base.count[idx];
> >  	spin_unlock(&mem->pcp_counter_lock);
> >  #endif
> > -	put_online_cpus();
> >  	return val;
> >  }
> >  
> > @@ -1284,6 +1282,7 @@ s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item)
> >  	struct mem_cgroup *iter;
> >  	s64 value;
> >  
> > +	get_online_cpus();
> >  	rcu_read_lock();
> >  	mem = mem_cgroup_from_task(current);
> >  	if (mem && !mem_cgroup_is_root(mem)) {
> > @@ -1305,6 +1304,7 @@ s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item)
> >  	} else
> >  		value = -EINVAL;
> >  	rcu_read_unlock();
> > +	put_online_cpus();
> >  
> >  	return value;
> >  }
> 
> Confused again.  There's no mem_cgroup_page_stat() in mainline,
> linux-next or in any patches in -mm.
>

Oops, sorry for the confusion. This patch applies on top of the dirty
limit patches posted by Greg. I should have posted this in response to
Greg's posting.

-- 
	Three Cheers,
	Balbir
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