Re: [PATCH] sched, mm: Optimize current_gfp_context()

[Waiman Long]

On 6/18/20 12:07 PM, Peter Zijlstra wrote:
> On Thu, Jun 18, 2020 at 11:58:47AM -0400, Waiman Long 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.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   include/linux/sched/mm.h | 10 ++++++----
> >   1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> > index 480a4d1b7dd8..09e01909d192 100644
> > --- 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 = current->flags;
> Per the above, I would've expected a READ_ONCE() there, otherwise the
> compiler can still re-load.
Right. Will do that.

Thanks,
Longman