xen-devel.lists.xenproject.org archive mirror
 help / color / mirror / Atom feed
From: Dario Faggioli <dfaggioli@suse.com>
To: xen-devel@lists.xenproject.org
Cc: George Dunlap <george.dunlap@citrix.com>,
	Ian Jackson <iwj@xenproject.org>
Subject: [Bugfix PATCH for-4.15] xen: credit2: fix per-entity load tracking when continuing running
Date: Fri, 19 Mar 2021 12:14:17 +0000	[thread overview]
Message-ID: <161615605709.5036.4052641880659992679.stgit@Wayrath> (raw)

If we schedule, and the current vCPU continues to run, its statistical
load is not properly updated, resulting in something like this, even if
all the 8 vCPUs are 100% busy:

(XEN) Runqueue 0:
(XEN) [...]
(XEN)   aveload            = 2097152 (~800%)
(XEN) [...]
(XEN)   Domain: 0 w 256 c 0 v 8
(XEN)     1: [0.0] flags=2 cpu=4 credit=9996885 [w=256] load=35 (~0%)
(XEN)     2: [0.1] flags=2 cpu=2 credit=9993725 [w=256] load=796 (~0%)
(XEN)     3: [0.2] flags=2 cpu=1 credit=9995885 [w=256] load=883 (~0%)
(XEN)     4: [0.3] flags=2 cpu=5 credit=9998833 [w=256] load=487 (~0%)
(XEN)     5: [0.4] flags=2 cpu=6 credit=9998942 [w=256] load=1595 (~0%)
(XEN)     6: [0.5] flags=2 cpu=0 credit=9994669 [w=256] load=22 (~0%)
(XEN)     7: [0.6] flags=2 cpu=7 credit=9997706 [w=256] load=0 (~0%)
(XEN)     8: [0.7] flags=2 cpu=3 credit=9992440 [w=256] load=0 (~0%)

As we can see, the average load of the runqueue as a whole is, instead,
computed properly.

This issue would, in theory, potentially affect Credit2 load balancing
logic. In practice, however, the problem only manifests (at least with
these characteristics) when there is only 1 runqueue active in the
cpupool, which also means there is no need to do any load-balancing.

Hence its real impact is pretty much limited to wrong per-vCPU load
percentages, when looking at the output of the 'r' debug-key.

With this patch, the load is updated and displayed correctly:

(XEN) Runqueue 0:
(XEN) [...]
(XEN)   aveload            = 2097152 (~800%)
(XEN) [...]
(XEN) Domain info:
(XEN)   Domain: 0 w 256 c 0 v 8
(XEN)     1: [0.0] flags=2 cpu=4 credit=9995584 [w=256] load=262144 (~100%)
(XEN)     2: [0.1] flags=2 cpu=6 credit=9992992 [w=256] load=262144 (~100%)
(XEN)     3: [0.2] flags=2 cpu=3 credit=9998918 [w=256] load=262118 (~99%)
(XEN)     4: [0.3] flags=2 cpu=5 credit=9996867 [w=256] load=262144 (~100%)
(XEN)     5: [0.4] flags=2 cpu=1 credit=9998912 [w=256] load=262144 (~100%)
(XEN)     6: [0.5] flags=2 cpu=2 credit=9997842 [w=256] load=262144 (~100%)
(XEN)     7: [0.6] flags=2 cpu=7 credit=9994623 [w=256] load=262144 (~100%)
(XEN)     8: [0.7] flags=2 cpu=0 credit=9991815 [w=256] load=262144 (~100%)

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
---
Cc: George Dunlap <george.dunlap@citrix.com>
Cc: Ian Jackson <iwj@xenproject.org>
---
Despite the limited effect, it's a bug. So:
- it should be backported;
- I think it should be included in 4.15. The risk is pretty low, for
  the same reasons already explained when describing its limited impact.
---
 xen/common/sched/credit2.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
index eb5e5a78c5..b3b5de94cf 100644
--- a/xen/common/sched/credit2.c
+++ b/xen/common/sched/credit2.c
@@ -3646,6 +3646,8 @@ static void csched2_schedule(
             runq_remove(snext);
             __set_bit(__CSFLAG_scheduled, &snext->flags);
         }
+        else
+            update_load(ops, rqd, snext, 0, now);
 
         /* Clear the idle mask if necessary */
         if ( cpumask_test_cpu(sched_cpu, &rqd->idle) )




             reply	other threads:[~2021-03-19 12:14 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-19 12:14 Dario Faggioli [this message]
2021-04-27  8:35 ` Ping: " Jan Beulich
2021-05-28 15:18   ` Dario Faggioli
2021-06-07 11:44 ` George Dunlap

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=161615605709.5036.4052641880659992679.stgit@Wayrath \
    --to=dfaggioli@suse.com \
    --cc=george.dunlap@citrix.com \
    --cc=iwj@xenproject.org \
    --cc=xen-devel@lists.xenproject.org \
    --subject='Re: [Bugfix PATCH for-4.15] xen: credit2: fix per-entity load tracking when continuing running' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).