Messages in this thread | | | From | Waiman Long <> | Subject | Re: [PATCH v3 5/5] mm/memcg: Optimize user context object stock access | Date | Thu, 15 Apr 2021 14:16:17 -0400 |
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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@redhat.com> >> Acked-by: Roman Gushchin <guro@fb.com> >> Reviewed-by: Shakeel Butt <shakeelb@google.com> > 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
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