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.
> if (objcg == stock->cached_objcg && stock->nr_bytes >= nr_bytes) {
> @@ -3201,7 +3241,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> ret = true;
> }
>
> - local_irq_restore(flags);
> + put_obj_stock(flags);
>
> return ret;
> }
> @@ -3254,8 +3294,13 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> {
> struct mem_cgroup *memcg;
>
> - if (stock->obj.cached_objcg) {
> - memcg = obj_cgroup_memcg(stock->obj.cached_objcg);
> + if (in_task() && stock->task_obj.cached_objcg) {
> + memcg = obj_cgroup_memcg(stock->task_obj.cached_objcg);
> + if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
> + return true;
> + }
> + if (stock->irq_obj.cached_objcg) {
> + memcg = obj_cgroup_memcg(stock->irq_obj.cached_objcg);
> if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
> return true;
> }
> @@ -3283,9 +3328,9 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> {
> unsigned long flags;
>
> - local_irq_save(flags);
> + get_obj_stock(flags);
> __refill_obj_stock(objcg, nr_bytes);
> - local_irq_restore(flags);
> + put_obj_stock(flags);
> }
>
> static void __mod_obj_stock_state(struct obj_cgroup *objcg,
> @@ -3325,9 +3370,9 @@ void mod_obj_stock_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> {
> unsigned long flags;
>
> - local_irq_save(flags);
> + get_obj_stock(flags);
> __mod_obj_stock_state(objcg, pgdat, idx, nr);
> - local_irq_restore(flags);
> + put_obj_stock(flags);
> }
>
> int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
> @@ -3380,10 +3425,10 @@ void obj_cgroup_uncharge_mod_state(struct obj_cgroup *objcg, size_t size,
> {
> unsigned long flags;
>
> - local_irq_save(flags);
> + get_obj_stock(flags);
> __refill_obj_stock(objcg, size);
> __mod_obj_stock_state(objcg, pgdat, idx, -(int)size);
> - local_irq_restore(flags);
> + put_obj_stock(flags);
> }
>
> #endif /* CONFIG_MEMCG_KMEM */
> --
> 2.18.1
>
