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From: Dario Faggioli <dario.faggioli@citrix.com>
To: xen-devel@lists.xenproject.org
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Tim Deegan <tim@xen.org>, Julien Grall <julien.grall@arm.com>,
	Jan Beulich <jbeulich@suse.com>
Subject: [PATCH 01/15] xen: in do_softirq() sample smp_processor_id() once and for all.
Date: Thu, 01 Jun 2017 19:33:41 +0200
Message-ID: <149633842141.12814.20908893053492021.stgit@Solace.fritz.box> (raw)
In-Reply-To: <149633614204.12814.14390287626133023934.stgit@Solace.fritz.box>

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;
- 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>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <George.Dunlap@eu.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>
---
 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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Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-01 17:33 [PATCH 00/15] xen/tools: add tracing to various Xen subsystems Dario Faggioli
2017-06-01 17:33 ` Dario Faggioli [this message]
2017-06-07 14:38   ` [PATCH 01/15] xen: in do_softirq() sample smp_processor_id() once and for all Jan Beulich
2017-06-08 14:12     ` George Dunlap
2017-06-08 14:20   ` George Dunlap
2017-06-08 14:42     ` Jan Beulich
2017-06-01 17:33 ` [PATCH 02/15] xen: tracing: avoid checking tb_init_done multiple times Dario Faggioli
2017-06-01 17:53   ` Andrew Cooper
2017-06-01 23:08     ` Dario Faggioli
2017-06-07 14:46   ` Jan Beulich
2017-06-07 15:55     ` Dario Faggioli
2017-06-07 16:06       ` Jan Beulich
2017-06-08 14:34         ` George Dunlap
2017-06-08 14:37   ` George Dunlap
2017-06-01 17:33 ` [PATCH 03/15] xen/tools: tracing: several improvements on IRQs tracing Dario Faggioli
2017-06-01 18:02   ` Andrew Cooper
2017-06-01 23:12     ` Dario Faggioli
2017-06-07 15:05   ` Jan Beulich
2017-06-07 15:45     ` Dario Faggioli
2017-06-07 15:58       ` Jan Beulich
2017-06-08 14:53         ` George Dunlap
2017-06-08 15:34           ` Jan Beulich
2017-06-08 14:59   ` George Dunlap
2017-06-01 17:34 ` [PATCH 04/15] tools: xenalyze: fix dumping of PM_IDLE events Dario Faggioli
2017-06-08 15:06   ` George Dunlap
2017-06-01 17:34 ` [PATCH 05/15] xen: make it possible to disable tracing in Kconfig Dario Faggioli
2017-06-01 18:43   ` Andrew Cooper
2017-06-07 11:01     ` Julien Grall
2017-06-07 15:14   ` Jan Beulich
2017-06-08 15:16     ` George Dunlap
2017-06-08 15:35       ` Jan Beulich
2017-06-08 15:37         ` George Dunlap
2017-06-08 15:44           ` Jan Beulich
2017-06-08 15:17   ` George Dunlap
2017-06-01 17:34 ` [PATCH 06/15] xen: trace IRQ enabling/disabling Dario Faggioli
2017-06-01 19:08   ` Andrew Cooper
2017-06-01 23:42     ` Dario Faggioli
2017-06-08 15:51       ` George Dunlap
2017-06-08 16:05       ` Jan Beulich
2017-06-07 11:16   ` Julien Grall
2017-06-07 15:22     ` Dario Faggioli
2017-06-09 10:51       ` Julien Grall
2017-06-09 10:53         ` Julien Grall
2017-06-09 10:55         ` George Dunlap
2017-06-09 11:00           ` Julien Grall
2017-06-08 16:01   ` George Dunlap
2017-06-08 16:11     ` Dario Faggioli
2017-06-09 10:41   ` Jan Beulich
2017-06-01 17:34 ` [PATCH 07/15] tools: tracing: handle IRQs on/off events in xentrace and xenalyze Dario Faggioli
2017-06-13 15:58   ` George Dunlap
2017-06-01 17:34 ` [PATCH 08/15] xen: trace RCU behavior Dario Faggioli
2017-06-09 10:48   ` Jan Beulich
2017-06-13 16:05   ` George Dunlap
2017-06-01 17:34 ` [PATCH 09/15] tools: tracing: handle RCU events in xentrace and xenalyze Dario Faggioli
2017-06-13 16:12   ` George Dunlap
2017-06-01 17:34 ` [PATCH 10/15] xen: trace softirqs Dario Faggioli
2017-06-09 10:51   ` Jan Beulich
2017-06-01 17:34 ` [PATCH 11/15] tools: tracing: handle RCU events in xentrace and xenalyze Dario Faggioli
2017-06-01 17:35 ` [PATCH 12/15] xen: trace tasklets Dario Faggioli
2017-06-09 10:59   ` Jan Beulich
2017-06-09 11:17     ` Dario Faggioli
2017-06-09 11:29       ` Jan Beulich
2017-06-01 17:35 ` [PATCH 13/15] tools: tracing: handle tasklets events in xentrace and xenalyze Dario Faggioli
2017-06-01 17:35 ` [PATCH 14/15] xen: trace timers Dario Faggioli
2017-06-01 17:35 ` [PATCH 15/15] tools: tracing: handle timers events in xentrace and xenalyze Dario Faggioli
2017-06-07 14:13 ` [PATCH 00/15] xen/tools: add tracing to various Xen subsystems Konrad Rzeszutek Wilk
2017-06-08 16:45   ` Dario Faggioli
2017-06-13 16:34 ` George Dunlap

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