Re: [patch 1/2] x86_64 page fault NMI-safe

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

Peter Zijlstra wrote:
> On Fri, 2010-08-06 at 15:18 +0900, Masami Hiramatsu wrote:
>> Peter Zijlstra wrote:
>>> On Wed, 2010-08-04 at 10:45 -0400, Mathieu Desnoyers wrote:
>>>
>>>> How do you plan to read the data concurrently with the writer overwriting the
>>>> data while you are reading it without corruption ?
>>> I don't consider reading while writing (in overwrite mode) a valid case.
>>>
>>> If you want to use overwrite, stop the writer before reading it.
>> For example, would you like to read system audit log always after
>> stop the audit?
>>
>> NO, that's a most important requirement for tracers, especially for
>> system admins (they're the most important users of Linux) to check
>> the system health and catch system troubles.
>>
>> For performance measurement and checking hotspot, one-shot tracing
>> is enough. But it's just for developers. But for the real world
>> computing, Linux is just an OS, users want to run their system,
>> middleware and applications, without troubles. But when they hit
>> a trouble, they wanna shoot it ASAP.
>> The flight recorder mode is mainly for those users.
> 
> You cannot over-write and consistently read the buffer, that's plain
> impossible. With sub-buffers you can swivel a sub-buffer and
> consistently read that, but there is no guarantee the next sub-buffer
> you steal was indeed adjacent to the previous buffer you stole as that
> might have gotten over-written by the active writer while you were
> stealing the previous one.

Right, we cannot ensure that. In over-written mode, reader could lose
some data, because of overwriting by writers. (or writer may fails
to write new data on buffer in non-overwritten mode)
However, I think that doesn't mean this mode is completely useless.
If we can know when(where) the data was lost, the rest of data
is enough useful in some cases.

> If you want to snapshot buffers, do that, simply swivel the whole trace
> buffer, and continue tracing in a new one, then consume the old trace in
> a consistent manner.

Hmm, would that consume much more memory compared with sub-buffer
ring buffer if we have spare buffers?
Or, if allocating it after reader opens buffer, will it also slow
down the reader process?

> I really see no value in being able to read unrelated bits and pieces of
> a buffer.

I think there is a trade-off of perfect snapshot and consuming memory,
and it depends on use-case in many cases.

> 
> So no, I will _not_ support reading an over-write buffer while there is
> an active reader.
> 

I hope you to reconsider how over-write buffer is useful even if
it is far from perfect.

Thank you,

-- 
Masami HIRAMATSU
2nd Research Dept.
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com