Re: [PATCH 2/3] X86: add a generic API to let vdso code detect context switch

[Shaohua Li <shli@fb.com>]

On Wed, Dec 10, 2014 at 11:11:27AM -0800, Andy Lutomirski wrote:
> On Wed, Dec 10, 2014 at 10:51 AM, Shaohua Li <shli@fb.com> wrote:
> > On Wed, Dec 10, 2014 at 10:38:41AM -0800, Andy Lutomirski wrote:
> >> On Sun, Dec 7, 2014 at 7:03 PM, Shaohua Li <shli@fb.com> wrote:
> >> > vdso code can't disable preempt, so it can be preempted at any time.
> >> > This makes a challenge to implement specific features. This patch adds a
> >> > generic API to let vdso code detect context switch.
> >> >
> >> > With this patch, every cpu maintains a context switch count. The upper
> >> > bits of the count is the logical cpu id, so the count can't be identical
> >> > for any cpu. The low bits of the count will be increased for each
> >> > context switch. For a x86_64 cpu with 4096 cpus, the context switch will
> >> > be overflowed for 2^(64 - 12) context switch, which is a long time and can be
> >> > ignored. The change of the count in giving time can be used to detect if
> >> > context switch occurs.
> >>
> >> Why do you need those high bits?  I don't understand how you could
> >> possibly confuse one cpu's count with another's unless you fail to
> >> make sure that you're reading the same address both times.
> >>
> >> That being said, I don't like this patch.  I'm not sure I have a much
> >> better idea, though.  More thoughts in the 0/0 email to follow.
> > the vdso code doesn't disable preemption, so it can be migrated between
> > cpus at any time, the usage is:
> >
> > get_countext_switch_count (in cpu A)
> > do_something (in cpu B)
> > get_countext_switch_count (in cpu C)
> >
> > The cpu A, B, C could be completely different. We want to make sure
> > there is no preemption here and we use the context switch count to judge
> > this. If the high bits is ignored, the context switch count could be
> > identical even A != C, then our judgement using the switch count is
> > wrong.
> 
> Sure, but you could compare the cpu numbers, too.

Aha, makes sense.

Thanks,
Shaohua