On 2017-11-16 06:03, Thomas Gleixner wrote:
On Wed, 15 Nov 2017, Peter Zijlstra wrote:
On Mon, Nov 13, 2017 at 06:06:02PM +0800, Quan Xu wrote:
From: Yang Zhang <yang.zhang.wz@xxxxxxxxx>
Implement a generic idle poll which resembles the functionality
found in arch/. Provide weak arch_cpu_idle_poll function which
can be overridden by the architecture code if needed.
No, we want less of those magic hooks, not more.
Interrupts arrive which may not cause a reschedule in idle loops.
In KVM guest, this costs several VM-exit/VM-entry cycles, VM-entry
for interrupts and VM-exit immediately. Also this becomes more
expensive than bare metal. Add a generic idle poll before enter
real idle path. When a reschedule event is pending, we can bypass
the real idle path.
Why not do a HV specific idle driver?
If I understand the problem correctly then he wants to avoid the heavy
lifting in tick_nohz_idle_enter() in the first place, but there is already
an interesting quirk there which makes it exit early. See commit
3c5d92a0cfb5 ("nohz: Introduce arch_needs_cpu"). The reason for this commit
looks similar. But lets not proliferate that. I'd rather see that go away.
agreed.
Even we can get more benifit than commit 3c5d92a0cfb5 ("nohz: Introduce
arch_needs_cpu")
in kvm guest. I won't proliferate that..
But the irq_timings stuff is heading into the same direction, with a more
complex prediction logic which should tell you pretty good how long that
idle period is going to be and in case of an interrupt heavy workload this
would skip the extra work of stopping and restarting the tick and provide a
very good input into a polling decision.
interesting. I have tested with IRQ_TIMINGS related code, which seems
not working so far.
Also I'd like to help as much as I can.
This can be handled either in a HV specific idle driver or even in the
generic core code. If the interrupt does not arrive then you can assume
within the predicted time then you can assume that the flood stopped and
invoke halt or whatever.
That avoids all of that 'tunable and tweakable' x86 specific hackery and
utilizes common functionality which is mostly there already.
here is some sample code. Poll for a while before enter halt in
cpuidle_enter_state()
If I get a reschedule event, then don't try to enter halt. (I hope this
is the right direction as Peter mentioned in another email)
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -210,6 +210,13 @@ int cpuidle_enter_state(struct cpuidle_device *dev,
struct cpuidle_driver *drv,
target_state = &drv->states[index];
}
+#ifdef CONFIG_PARAVIRT
+ paravirt_idle_poll();
+
+ if (need_resched())
+ return -EBUSY;
+#endif
+
/* Take note of the planned idle state. */
sched_idle_set_state(target_state);
thanks,
Quan
Alibaba Cloud
