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.faggi...@citrix.com>
Reviewed-by: George Dunlap <george.dun...@eu.citrix.com>
---
Cc: Andrew Cooper <andrew.coop...@citrix.com>
Cc: Jan Beulich <jbeul...@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.w...@oracle.com>
Cc: Stefano Stabellini <sstabell...@kernel.org>
Cc: Julien Grall <julien.gr...@arm.com>
Cc: Tim Deegan <t...@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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