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

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]).

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:

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 

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."

Since then:
- changed "once and for all" with "only once", as requested by George (and
  applied his Reviewed-by, as he said I could).
 xen/common/softirq.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/xen/common/softirq.c b/xen/common/softirq.c
index ac12cf8..67c84ba 100644
--- a/xen/common/softirq.c
+++ b/xen/common/softirq.c
@@ -27,16 +27,12 @@ static DEFINE_PER_CPU(unsigned int, batching);
 static void __do_softirq(unsigned long ignore_mask)
-    unsigned int i, cpu;
+    unsigned int i, cpu = smp_processor_id();
     unsigned long pending;
     for ( ; ; )
-        /*
-         * Initialise @cpu on every iteration: SCHEDULE_SOFTIRQ may move
-         * us to another processor.
-         */
-        cpu = smp_processor_id();
+        ASSERT(cpu == smp_processor_id());
         if ( rcu_pending(cpu) )

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