Messages in this thread | | | Subject | Re: [PATCH v2] sched, mm: Optimize current_gfp_context() | From | Waiman Long <> | Date | Tue, 11 Aug 2020 19:32:52 -0400 |
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On 8/11/20 6:29 PM, Andrew Morton wrote: > 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?
It was a change suggested by PeterZ. He said without the READ_ONCE(), the compiler may still choose to access current->flags multiple times instead of just reading it once. I think he is right and so I made the change.
Cheers, Longman
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