Re: [PATCH v3 5/5] mm/memcg: Optimize user context object stock access

[Waiman Long]

On 4/15/21 1:53 PM, Johannes Weiner wrote:
> On Tue, Apr 13, 2021 at 09:20:27PM -0400, Waiman Long wrote:
> > Most kmem_cache_alloc() calls are from user context. With instrumentation
> > enabled, the measured amount of kmem_cache_alloc() calls from non-task
> > context was about 0.01% of the total.
> >
> > The irq disable/enable sequence used in this case to access content
> > from object stock is slow.  To optimize for user context access, there
> > are now two object stocks for task context and interrupt context access
> > respectively.
> >
> > The task context object stock can be accessed after disabling preemption
> > which is cheap in non-preempt kernel. The interrupt context object stock
> > can only be accessed after disabling interrupt. User context code can
> > access interrupt object stock, but not vice versa.
> >
> > The mod_objcg_state() function is also modified to make sure that memcg
> > and lruvec stat updates are done with interrupted disabled.
> >
> > The downside of this change is that there are more data stored in local
> > object stocks and not reflected in the charge counter and the vmstat
> > arrays.  However, this is a small price to pay for better performance.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > Acked-by: Roman Gushchin <guro@xxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> This makes sense, and also explains the previous patch a bit
> better. But please merge those two.
The reason I broke it into two is so that the patches are individually 
easier to review. I prefer to update the commit log of patch 4 to 
explain why the obj_stock structure is introduced instead of merging the 
two.
> > @@ -2229,7 +2229,8 @@ struct obj_stock {
> >   struct memcg_stock_pcp {
> >   	struct mem_cgroup *cached; /* this never be root cgroup */
> >   	unsigned int nr_pages;
> > -	struct obj_stock obj;
> > +	struct obj_stock task_obj;
> > +	struct obj_stock irq_obj;
> >   
> >   	struct work_struct work;
> >   	unsigned long flags;
> > @@ -2254,11 +2255,48 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> >   }
> >   #endif
> >   
> > +/*
> > + * Most kmem_cache_alloc() calls are from user context. The irq disable/enable
> > + * sequence used in this case to access content from object stock is slow.
> > + * To optimize for user context access, there are now two object stocks for
> > + * task context and interrupt context access respectively.
> > + *
> > + * The task context object stock can be accessed by disabling preemption only
> > + * which is cheap in non-preempt kernel. The interrupt context object stock
> > + * can only be accessed after disabling interrupt. User context code can
> > + * access interrupt object stock, but not vice versa.
> > + */
> >   static inline struct obj_stock *current_obj_stock(void)
> >   {
> >   	struct memcg_stock_pcp *stock = this_cpu_ptr(&memcg_stock);
> >   
> > -	return &stock->obj;
> > +	return in_task() ? &stock->task_obj : &stock->irq_obj;
> > +}
> > +
> > +#define get_obj_stock(flags)				\
> > +({							\
> > +	struct memcg_stock_pcp *stock;			\
> > +	struct obj_stock *obj_stock;			\
> > +							\
> > +	if (in_task()) {				\
> > +		preempt_disable();			\
> > +		(flags) = -1L;				\
> > +		stock = this_cpu_ptr(&memcg_stock);	\
> > +		obj_stock = &stock->task_obj;		\
> > +	} else {					\
> > +		local_irq_save(flags);			\
> > +		stock = this_cpu_ptr(&memcg_stock);	\
> > +		obj_stock = &stock->irq_obj;		\
> > +	}						\
> > +	obj_stock;					\
> > +})
> > +
> > +static inline void put_obj_stock(unsigned long flags)
> > +{
> > +	if (flags == -1L)
> > +		preempt_enable();
> > +	else
> > +		local_irq_restore(flags);
> >   }
> Please make them both functions and use 'unsigned long *flags'.
Sure, I can do that.
> Also I'm not sure doing in_task() twice would actually be more
> expensive than the == -1 special case, and easier to understand.
I can make that change too. Either way is fine with me.
> > @@ -2327,7 +2365,9 @@ static void drain_local_stock(struct work_struct *dummy)
> >   	local_irq_save(flags);
> >   
> >   	stock = this_cpu_ptr(&memcg_stock);
> > -	drain_obj_stock(&stock->obj);
> > +	drain_obj_stock(&stock->irq_obj);
> > +	if (in_task())
> > +		drain_obj_stock(&stock->task_obj);
> >   	drain_stock(stock);
> >   	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
> >   
> > @@ -3183,7 +3223,7 @@ static inline void mod_objcg_state(struct obj_cgroup *objcg,
> >   	memcg = obj_cgroup_memcg(objcg);
> >   	if (pgdat)
> >   		lruvec = mem_cgroup_lruvec(memcg, pgdat);
> > -	__mod_memcg_lruvec_state(memcg, lruvec, idx, nr);
> > +	mod_memcg_lruvec_state(memcg, lruvec, idx, nr);
> >   	rcu_read_unlock();
> This is actually a bug introduced in the earlier patch, isn't it?
> Calling __mod_memcg_lruvec_state() without irqs disabled...
Not really, in patch 3, mod_objcg_state() is called only in the stock 
update context where interrupt had already been disabled. But now, that 
is no longer the case, that is why i need to update mod_objcg_state() to 
make sure irq is disabled before updating vmstat data array.

Cheers,
Longman