Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this isfor x86_64 right now)

[H. Peter Anvin]

On 02/18/2011 02:39 PM, Andi Kleen wrote:
> On Fri, Feb 18, 2011 at 03:10:18PM -0500, Dominique Toupin wrote:
>>
>> My understanding is stop_machine will stop all processors for many ms.
> 
> I haven't measured it recently, but as long as the callback inside stop
> machine is short it definitely shouldn't be "many ms". The latency
> is bound by how long each CPU needs to answer to an interrupt, so if
> you have some code that disables interrupts for a long time it will take
> long -- but then your realtime response will be already bad. 
> 
> The interrupts are also done in parallel, so the interrupt latencies
> don't add up.
> 
> If all the CPUs answer in a reasonable time it's still not a cheap
> operation, but nothing that takes "many ms". Most likely it's fine
> for most soft real time purposes.
> 

We should also be able to use the breakpoint hack to avoid holding all
the CPUs.  They still need to be interrupted, but that skips the
rendezvous operation.

	-hpa

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