[lkml]   [2015]   [Apr]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler
Hi Ingo,

On Wed, 2015-04-15 at 09:46 +0200, Ingo Molnar wrote:
> * Steven Rostedt <> wrote:
> > You are correct. Now I'm thinking that the WRITE_ONCE() is not needed,
> > and just a:
> >
> > p->mm->numa_scan_seq = READ_ONCE(p->numa_scan_seq) + 1;
> >
> > Can be done. But I'm still trying to wrap my head around why this is
> > needed here. Comments would have been really helpful. We should make
> > all READ_ONCE() WRITE_ONCE and obsolete ACCESS_ONCE() have mandatory
> > comments just like we do with memory barriers.
> So the original ACCESS_ONCE() barriers were misguided to begin with: I
> think they tried to handle races with the scheduler balancing softirq
> and tried to avoid having to use atomics for the sequence counter
> (which would be overkill), but things like ACCESS_ONCE(x)++ never
> guaranteed atomicity (or even coherency) of the update.
> But since in reality this is only statistical sampling code, all these
> compiler barriers can be removed I think. Peter, Mel, Rik, do you
> agree?

Though in the read side for accessing things such as numa_scan_seq, we
still want to keep them in order to guarantee that numa_scan_seq is only
loaded once right?

static void task_numa_placement(struct task_struct *p)

seq = ACCESS_ONCE(p->mm->numa_scan_seq);
if (p->numa_scan_seq == seq)
p->numa_scan_seq = seq;


 \ /
  Last update: 2015-04-16 05:21    [W:0.108 / U:2.524 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site