Date:   Wed, 30 May 2018 16:27:52 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        kernel test robot <xiaolong.ye@...el.com>, lkp@...org,
        linux-kernel@...r.kernel.org
Subject: Re: [LKP] [lkp-robot] [mm] e27be240df: will-it-scale.per_process_ops
 -27.2% regression

On Tue, May 29, 2018 at 10:48:16AM +0200, Michal Hocko wrote:
> On Mon 28-05-18 16:52:01, Aaron Lu wrote:
> > On Tue, May 08, 2018 at 01:26:40PM -0400, Johannes Weiner wrote:
> > > Hello,
> > > 
> > > On Tue, May 08, 2018 at 01:34:51PM +0800, kernel test robot wrote:
> > > > FYI, we noticed a -27.2% regression of will-it-scale.per_process_ops due to commit:
> > > > 
> > > > 
> > > > commit: e27be240df53f1a20c659168e722b5d9f16cc7f4 ("mm: memcg: make sure memory.events is uptodate when waking pollers")
> > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > > > 
> > > > in testcase: will-it-scale
> > > > on test machine: 72 threads Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz with 128G memory
> > > > with following parameters:
> > > > 
> > > > 	nr_task: 100%
> > > > 	mode: process
> > > > 	test: page_fault3
> > > > 	cpufreq_governor: performance
> > > > 
> > > > test-description: Will It Scale takes a testcase and runs it from 1 through to n parallel copies to see if the testcase will scale. It builds both a process and threads based test in order to see any differences between the two.
> > > > test-url: https://github.com/antonblanchard/will-it-scale
> > > 
> > > This is surprising. Do you run these tests in a memory cgroup with a
> > > limit set? Can you dump that cgroup's memory.events after the run?
> > 
> > "Some background in case it's forgotten: we do not set any memory control
> > group specifically and the test machine is using ramfs as its root.
> > The machine has plenty memory, no swap is setup. All pages belong to
> > root_mem_cgroup"
> > 
> > Turned out the performance change is due to 'struct mem_cgroup' layout
> > change, i.e. if I do:
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index d99b71bc2c66..c767db1da0bb 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -205,7 +205,6 @@ struct mem_cgroup {
> >  	int		oom_kill_disable;
> >  
> >  	/* memory.events */
> > -	atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
> >  	struct cgroup_file events_file;
> >  
> >  	/* protect arrays of thresholds */
> > @@ -238,6 +237,7 @@ struct mem_cgroup {
> >  	struct mem_cgroup_stat_cpu __percpu *stat_cpu;
> >  	atomic_long_t		stat[MEMCG_NR_STAT];
> >  	atomic_long_t		events[NR_VM_EVENT_ITEMS];
> > +	atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
> >  
> >  	unsigned long		socket_pressure;
> 
> Well, I do not mind moving memory_events down to other stats/events. I
> suspect Johannes' chosen the current location to be close to
> events_file.
> 
> > The performance will restore.
> > 
> > Move information:
> > With this patch, perf profile+annotate showed increased cycles spent on
> > accessing root_mem_cgroup->stat_cpu in count_memcg_event_mm()(called by
> > handle_mm_fault()):
> > 
> >        │             x = count + __this_cpu_read(memcg->stat_cpu->events[idx]);
> >  92.31 │       mov    0x308(%rcx),%rax
> >   0.58 │       mov    %gs:0x1b0(%rax),%rdx
> >   0.09 │       add    $0x1,%rdx
> > 
> > And in __mod_memcg_state() called by page_add_file_rmap():
> > 
> >        │             x = val + __this_cpu_read(memcg->stat_cpu->count[idx]);
> >  70.89 │       mov    0x308(%rdi),%rdx
> >   0.43 │       mov    %gs:0x68(%rdx),%rax
> >   0.08 │       add    %rbx,%rax
> >        │             if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
> > 
> > My first reaction is, with the patch changing the sturcture layout, the
> > stat_cpu field might end up in a cacheline that is constantly being
> > written to. With the help of pahole, I got:
> > 1 after this patch(bad)
> >        
> >         /* --- cacheline 12 boundary (768 bytes) --- */
> >         long unsigned int          move_lock_flags;      /*   768     8 */
> >         struct mem_cgroup_stat_cpu * stat_cpu;           /*   776     8 */
> >         atomic_long_t              stat[34];             /*   784     0 */
> > 
> > stat[0] - stat[5] falls in this cacheline.
> > 
> > 2 before this patch(good)
> >         /* --- cacheline 11 boundary (704 bytes) was 8 bytes ago --- */
> >         long unsigned int          move_charge_at_immigrate; /*   712     8 */
> >         atomic_t                   moving_account;       /*   720     0 */
> > 
> >         /* XXX 4 bytes hole, try to pack */
> > 
> >         spinlock_t                 move_lock;            /*   724     0 */
> > 
> >         /* XXX 4 bytes hole, try to pack */
> > 
> >         struct task_struct *       move_lock_task;       /*   728     8 */
> >         long unsigned int          move_lock_flags;      /*   736     8 */
> >         struct mem_cgroup_stat_cpu * stat_cpu;           /*   744     8 */
> >         atomic_long_t              stat[34];             /*   752     0 */
> > 
> > stat[0] - stat[1] falls in this cacheline.
> > 
> > We now have more stat[]s fall in the cacheline, but then I realized
> > stats[0] - stat[12] are never written to for a memory control group, the
> > first written field is 13(NR_FILE_MAPPED).
> 
> This is a bit scary though. Seeing 27% regression just because of this
> is really unexpected and fragile wrt. future changes.

Agree.

And it turned out as long as moving_account, move_lock_task and stat_cpu
being placed in the same cacheline, the performance will restore.

moving_account is accessed in lock_page_memcg(), move_lock_task is
accessed in unlock_page_memcg() and stat_cpu is accessed in multiple
places that calls __mod_memcg_state(). Placing the 3 fields in the same
cacheline really helps this workload.

>  
> > So I think my first reaction is wrong.
> > 
> > Looking at the good layout, there is a field moving_account that will be
> > accessed during the test in lock_page_memcg(), and that access is always
> > read only since there is no page changing memcg. So the good performance
> > might be due to having the two fields in the cache line. I moved the
> > moving_account field to the same cacheline as stat_cpu for the bad case,
> > the performance restored a lot but still not as good as base.
> > 
> > I'm not sure where to go next step and would like to seek some
> > suggestion. Based on my analysis, it appears the good performance for
> > base is entirely by accident(having moving_account and stat_cpu in the
> > same cacheline), we never ensure that. In the meantime, it might not be
> > a good idea to ensure that since stat_cpu should be an always_read field
> > while moving_account will be modified when needed.
> > 
> > Or any idea what might be the cause? Thanks.
> 
> Can you actually prepare a patch with all these numbers and a big fat
> comment in the structure to keep the most hot counters close to
> moving_account. Maybe we want to re-organize this some more and pull
> move_lock* out of the sensitive cache line because they are a slow path
> stuff. We would have more stasts in the same cache line then. What do
> you think?

Below is what I did to get the performance back for this workload, will
need to verify how it works with the other one([LKP] [lkp-robot] [mm,
memcontrol] 309fe96bfc: vm-scalability.throughput +23.0% improvement):

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index d99b71bc2c66..c79972a78d6c 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -158,6 +158,15 @@ enum memcg_kmem_state {
 	KMEM_ONLINE,
 };
 
+#if defined(CONFIG_SMP)
+struct memcg_padding {
+	char x[0];
+} ____cacheline_internodealigned_in_smp;
+#define MEMCG_PADDING(name)      struct memcg_padding name;
+#else
+#define MEMCG_PADDING(name)
+#endif
+
 /*
  * The memory controller data structure. The memory controller controls both
  * page cache and RSS per cgroup. We would eventually like to provide
@@ -225,17 +234,23 @@ struct mem_cgroup {
 	 * mem_cgroup ? And what type of charges should we move ?
 	 */
 	unsigned long move_charge_at_immigrate;
+	/* taken only while moving_account > 0 */
+	spinlock_t		move_lock;
+	unsigned long		move_lock_flags;
+
+	MEMCG_PADDING(_pad1_);
+
 	/*
 	 * set > 0 if pages under this cgroup are moving to other cgroup.
 	 */
 	atomic_t		moving_account;
-	/* taken only while moving_account > 0 */
-	spinlock_t		move_lock;
 	struct task_struct	*move_lock_task;
-	unsigned long		move_lock_flags;
 
 	/* memory.stat */
 	struct mem_cgroup_stat_cpu __percpu *stat_cpu;
+
+	MEMCG_PADDING(_pad2_);
+
 	atomic_long_t		stat[MEMCG_NR_STAT];
 	atomic_long_t		events[NR_VM_EVENT_ITEMS];
 

I'm not entirely sure if this is the right thing to do, considering
move_lock_task and moving_account could be modified while stat_cpu will
remain unchanged. But I think it is a rare case for move_lock_task and
moving_account to be modified? Per my understanding, only when a task
moves to another memory cgroup will those fields being modified. If so,
I think the above diff should be OK. Thoughts?

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