Messages in this thread | | | Date | Fri, 29 Feb 2008 10:24:13 -0800 | From | Max Krasnyanskiy <> | Subject | Re: [patch 1/2] add ALL_CPUS option to stop_machine_run() |
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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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