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

Andy Lutomirski
2014-12-10T19:11:51Z

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.

--Andy

>
> Thanks,
> Shaohua

--
Andy Lutomirski
AMA Capital Management, LLC