Re: [Xen-devel] [PATCH 01/15] xen: in do_softirq() sample smp_processor_id() once and for all.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.06.17 at 19:33, <dario.faggioli@xxxxxxxxxx> wrote:
> In fact, right now, we read it at every iteration of the loop.
> The reason it's done like this is how context switch was handled
> on IA64 (see commit ae9bfcdc, "[XEN] Various softirq cleanups" [1]).
> 
> However:
> 1) we don't have IA64 any longer, and all the achitectures that
>    we do support, are ok with sampling once and for all;
> 2) sampling at every iteration (slightly) affect performance;
> 3) sampling at every iteration is misleading, as it makes people
>    believe that it is currently possible that SCHEDULE_SOFTIRQ
>    moves the execution flow on another CPU (and the comment,
>    by reinforcing this belief, makes things even worse!).
> 
> Therefore, let's:
> - do the sampling once and for all, and remove the comment;
> - leave an ASSERT() around, so that, if context switching
>   logic changes (in current or new arches), we will notice.

I'm not convinced. The comment clearly says "may", and I don't
think the performance overhead of smp_processor_id() is that
high. Keeping the code as is allows some future port to do
context switches like ia64 did, if that's more efficient there.

Jan


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