Re: [Xen-devel] [PATCH v2 1/6] xen: in do_softirq() sample smp_processor_id() once and for all.

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, Aug 16, 2017 at 5:45 PM, Dario Faggioli
<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 only once, and remove the comment;
> - leave an ASSERT() around, so that, if context switching
>   logic changes (in current or new arches), we will notice.
>
> [1] Some more (historical) information here:
>     
> http://old-list-archives.xenproject.org/archives/html/xen-devel/2006-06/msg01262.html
>
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> ---
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> Cc: Tim Deegan <tim@xxxxxxx>
> ---
> This has been submitted already, as a part of another series. Discussion is 
> here:
>  https://lists.xen.org/archives/html/xen-devel/2017-06/msg00102.html
>
> For the super lazy, Jan's latest word in that thread were these:
>  "I've voiced my opinion, but I don't mean to block the patch. After
>   all there's no active issue the change introduces."
>  (https://lists.xen.org/archives/html/xen-devel/2017-06/msg00797.html)
>
> Since then:
> - changed "once and for all" with "only once", as requested by George (and
>   applied his Reviewed-by, as he said I could).


The commit message, but forgot to change the title. :-)  That can be
addressed on check-in if need be.

 -George

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