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SubjectRe: [patch 1/2] add ALL_CPUS option to stop_machine_run()
Ingo Molnar wrote:
> * Max Krasnyanskiy <maxk@qualcomm.com> wrote:
>
>>> -allow stop_mahcine_run() to call a function on all cpus. Calling
>>> stop_machine_run() with a 'ALL_CPUS' invokes this new behavior.
>>> stop_machine_run() proceeds as normal until the calling cpu has
>>> invoked 'fn'. Then, we tell all the other cpus to call 'fn'.
>> Jason, we're actually trying to reduce the usage of the stop_machine
>> in general. [...]
>
> please talk in your own name. Stop-machine is a very elegant tool that
> simplifies a lot of hard things in the kernel and is reasonably fast as
> well. We've just recently added two new usages of it and more are
> planned.
>
> _you_ might be the one who wants to 'reduce the usage of stop_machine' -
> but that means it is _you_ who first has to convert a number of very
> difficult pieces of code to "something else".
Sure I started the discussion but I suppose you missed Andi's and other
replies. All I said that people should think twice before relying on it.
btw I'm ok if I _am_ the _one_ who has to convert those pieces of code, that's
part of the fun :). But if people keep adding stuff which uses stom_machine
that may be pretty difficult :).

btw Being an RT guy you do not think that stop machine is evil ? I mean from
the overhead and especially latency perspective. By overhead I mean if you
have 100+ cpu box that Paul and other guys have mentioned here. Every single
CPU has to be frozen. You said it's reasonably fast. I guess it depends what's
reasonable. And from the latency perspective all bets are off. We have no
guaranties whatsoever as to hold long it will take for cpu X to get frozen
(there numerous factors here) and all the other cpus have to wait for it.
As I said for some things there is just no other way but to use the
stop_machine but we should try to minimize that as much as possible.

Max



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