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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.06.17 at 16:20, <george.dunlap@xxxxxxxxxx> wrote:
> On 01/06/17 18:33, Dario Faggioli 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;
> 
> "Once and for all" has a much stronger sense of finality than you're
> using here: reading smp_processor_id() in that function "once and for
> all" would mean you would never have to read it on that function, on
> that particular core, ever again, until the end of the universe. :-)
> 
> I think I'd say, "only once".  The scope of the "only" in that case is
> "this function call", not "all calls forever".
> 
> With that changed:
> 
> Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> 
> (Although we till need Jan's acquiescence.)

I've voiced my opinion, but I don't mean to block the patch. After
all there's no active issue the change introduces.

Jan


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