| Message ID | 150114249133.22910.6287911784333237605.stgit@Solace (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
>>> Dario Faggioli <dario.faggioli@citrix.com> 07/27/17 10:01 AM >>> >In this commit, we implement the above, using rcu_needs_cpu(), >in a way similar to how it is used in Linux. This it correct, >useful and not wasteful for CPUs that participate in grace >period, but have not a callback queued. For the ones that >has callbacks, an optimization that avoids having to spin is >introduced in next commit. As always, you can't know what will be the "next commit". Please use e.g. "in a subsequent change". Other than that Reviewed-by: Jan Beulich <jbeulich@suse.com> Jan
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h index 6673b27..df93a86 100644 --- a/xen/include/xen/sched.h +++ b/xen/include/xen/sched.h @@ -842,7 +842,8 @@ uint64_t get_cpu_idle_time(unsigned int cpu); /* * Used by idle loop to decide whether there is work to do: - * (1) Run softirqs; or (2) Play dead; or (3) Run tasklets. + * (1) Deal with RCU; (2) or run softirqs; or (3) Play dead; + * or (4) Run tasklets. * * About (3), if a tasklet is enqueued, it will be scheduled * really really soon, and hence it's pointless to try to @@ -850,7 +851,8 @@ uint64_t get_cpu_idle_time(unsigned int cpu); * the tasklet_work_to_do() helper). */ #define cpu_is_haltable(cpu) \ - (!softirq_pending(cpu) && \ + (!rcu_needs_cpu(cpu) && \ + !softirq_pending(cpu) && \ cpu_online(cpu) && \ !per_cpu(tasklet_work_to_do, cpu))
If a CPU has a callback queued, it must be ready to invoke it, as soon as all the other CPUs involved in the grace period has gone through a quiescent state. But if we let such CPU go idle, we can't really tell when (if!) it will realize that it is actually time to invoke the callback. To solve this problem, a CPU that has a callback queued (and has already gone through a quiescent state itself) will stay online, until the grace period ends, and the callback can be invoked. This is similar to what Linux does, and is the second and last step for fixing the overly long (or infinite!) grace periods. The problem, though, is that, within Linux, we have the tick, so, all that is necessary is to not stop the tick for the CPU (even if it has gone idle). In Xen, there's no tick, so we must avoid for the CPU to go idle entirely, and let it spin on rcu_pending(), consuming power and causing overhead. In this commit, we implement the above, using rcu_needs_cpu(), in a way similar to how it is used in Linux. This it correct, useful and not wasteful for CPUs that participate in grace period, but have not a callback queued. For the ones that has callbacks, an optimization that avoids having to spin is introduced in next commit. 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: Ian Jackson <ian.jackson@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> Cc: Wei Liu <wei.liu2@citrix.com> --- xen/include/xen/sched.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)