[Xen-changelog] [xen stable-4.6] xen: credit1: return the 'time remaining to the limit' as next timeslice.

[patchbot@xxxxxxx]

commit 92848cf9903c10a9585be6ac1fb1316c25b8dad6
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Tue Oct 25 17:15:35 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Oct 25 17:15:35 2016 +0200

    xen: credit1: return the 'time remaining to the limit' as next timeslice.
    
    If vcpu x has run for 200us, and sched_ratelimit_us is
    1000us, continue running x _but_ return 1000us-200us as
    the next time slice. This way, next scheduling point will
    happen in 800us, i.e., exactly at the point when x crosses
    the threshold, and can be descheduled (if appropriate).
    
    Right now (without this patch), we're always returning
    sched_ratelimit_us (1000us, in the example above), which
    means we're (potentially) allowing x to run more than
    it should have been able to.
    
    Note that, however, in order to avoid setting timers to very
    short intervals, which is part of the purpose of rate limiting,
    we never use a time slice smaller than a well defined threshold.
    Such threshold (CSCHED_MIN_TIMER defined in this patch) is, in
    general independent from rate limiting, but it looks a good idea
    to set it to the minimum possible ratelimiting value.
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    master commit: 0053127890ebe9cafbd232752636a15881e4915a
    master date: 2016-09-30 14:46:36 +0100
---
 xen/common/sched_credit.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 2716603..2337ead 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -50,6 +50,8 @@
 /* Default timeslice: 30ms */
 #define CSCHED_DEFAULT_TSLICE_MS    30
 #define CSCHED_CREDITS_PER_MSEC     10
+/* Never set a timer shorter than this value. */
+#define CSCHED_MIN_TIMER            XEN_SYSCTL_SCHED_RATELIMIT_MIN
 
 
 /*
@@ -1713,7 +1715,15 @@ csched_schedule(
         snext = scurr;
         snext->start_time += now;
         perfc_incr(delay_ms);
-        tslice = MICROSECS(prv->ratelimit_us);
+        /*
+         * Next timeslice must last just until we'll have executed for
+         * ratelimit_us. However, to avoid setting a really short timer, which
+         * will most likely be inaccurate and counterproductive, we never go
+         * below CSCHED_MIN_TIMER.
+         */
+        tslice = MICROSECS(prv->ratelimit_us) - runtime;
+        if ( unlikely(runtime < CSCHED_MIN_TIMER) )
+            tslice = CSCHED_MIN_TIMER;
         ret.migrated = 0;
         goto out;
     }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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