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

[Dario Faggioli <dario.faggioli@citrix.com>]

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@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Tim Deegan <tim@xen.org>
---
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).
---
 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) )
             rcu_check_callbacks(cpu);


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