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Date: Mon, 12 Apr 2021 21:03:00 -0400
From: Waiman Long <llong@...hat.com>
To: Shakeel Butt <shakeelb@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Vladimir Davydov <vdavydov.dev@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Vlastimil Babka <vbabka@...e.cz>, Roman Gushchin <guro@...com>,
LKML <linux-kernel@...r.kernel.org>,
Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
Muchun Song <songmuchun@...edance.com>,
Alex Shi <alex.shi@...ux.alibaba.com>,
Chris Down <chris@...isdown.name>,
Yafang Shao <laoar.shao@...il.com>,
Wei Yang <richard.weiyang@...il.com>,
Masayoshi Mizuma <msys.mizuma@...il.com>,
Xing Zhengjun <zhengjun.xing@...ux.intel.com>
Subject: Re: [PATCH v2 5/5] mm/memcg: Optimize user context object stock
access
On 4/12/21 7:10 PM, Shakeel Butt wrote:
> On Mon, Apr 12, 2021 at 3:55 PM Waiman Long <longman@...hat.com> 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@...hat.com>
>> Acked-by: Roman Gushchin <guro@...com>
>> ---
>> 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
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