Messages in this thread | | | Date | Tue, 11 Aug 2020 15:29:21 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH v2] sched, mm: Optimize current_gfp_context() |
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On Thu, 18 Jun 2020 17:29:36 -0400 Waiman Long <longman@redhat.com> wrote:
> The current_gfp_context() converts a number of PF_MEMALLOC_* per-process > flags into the corresponding GFP_* flags for memory allocation. In > that function, current->flags is accessed 3 times. That may lead to > duplicated access of the same memory location. > > This is not usually a problem with minimal debug config options on as the > compiler can optimize away the duplicated memory accesses. With most > of the debug config options on, however, that may not be the case. > For example, the x86-64 object size of the __need_fs_reclaim() in a > debug kernel that calls current_gfp_context() was 309 bytes. With this > patch applied, the object size is reduced to 202 bytes. This is a saving > of 107 bytes and will probably be slightly faster too. > > ... > > --- a/include/linux/sched/mm.h > +++ b/include/linux/sched/mm.h > @@ -181,18 +181,20 @@ static inline bool in_vfork(struct task_struct *tsk) > */ > static inline gfp_t current_gfp_context(gfp_t flags) > { > - if (unlikely(current->flags & > + unsigned int pflags = READ_ONCE(current->flags);
Why use READ_ONCE() here?
> + if (unlikely(pflags & > (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_NOCMA))) { > /* > * NOIO implies both NOIO and NOFS and it is a weaker context > * so always make sure it makes precedence > */ > - if (current->flags & PF_MEMALLOC_NOIO) > + if (pflags & PF_MEMALLOC_NOIO) > flags &= ~(__GFP_IO | __GFP_FS); > - else if (current->flags & PF_MEMALLOC_NOFS) > + else if (pflags & PF_MEMALLOC_NOFS) > flags &= ~__GFP_FS; > #ifdef CONFIG_CMA > - if (current->flags & PF_MEMALLOC_NOCMA) > + if (pflags & PF_MEMALLOC_NOCMA) > flags &= ~__GFP_MOVABLE; > #endif > }
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