Re: [PATCH v2 5/5] mm/memcg: Optimize user context object stock access

[Waiman Long]

On 4/12/21 7:10 PM, Shakeel Butt wrote:
> On Mon, Apr 12, 2021 at 3:55 PM Waiman Long <longman@xxxxxxxxxx> 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>
> > ---
> >   mm/memcontrol.c | 73 +++++++++++++++++++++++++++++++++++++++----------
> >   1 file changed, 59 insertions(+), 14 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 69f728383efe..29f2df76644a 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -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);     \
> The above line was missing in the previous version.
>
> > +               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);
> >   }
> >
> >   /**
> > @@ -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();
> >   }
> >
> > @@ -3193,7 +3233,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> >          unsigned long flags;
> >          bool ret = false;
> >
> > -       local_irq_save(flags);
> > +       stock = get_obj_stock(flags);
> >
> >          stock = current_obj_stock();
> The above is redundant.

Right. I should check the patch carefully. Will remove it.

Thanks,
Longman