Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenariosin PLE handler
From: Raghavendra K T
Date: Mon Sep 24 2012 - 09:33:29 EST
On 09/24/2012 06:06 PM, Peter Zijlstra wrote:
On Mon, 2012-09-24 at 17:22 +0530, Raghavendra K T wrote:
On 09/24/2012 05:04 PM, Peter Zijlstra wrote:
On Fri, 2012-09-21 at 17:29 +0530, Raghavendra K T wrote:
In some special scenarios like #vcpu<= #pcpu, PLE handler may
prove very costly, because there is no need to iterate over vcpus
and do unsuccessful yield_to burning CPU.
What's the costly thing? The vm-exit, the yield (which should be a nop
if its the only task there) or something else entirely?
Both vmexit and yield_to() actually,
because unsuccessful yield_to() overall is costly in PLE handler.
This is because when we have large guests, say 32/16 vcpus, and one
vcpu is holding lock, rest of the vcpus waiting for the lock, when they
do PL-exit, each of the vcpu try to iterate over rest of vcpu list in
the VM and try to do directed yield (unsuccessful). (O(n^2) tries).
this results is fairly high amount of cpu burning and double run queue
lock contention.
(if they were spinning probably lock progress would have been faster).
As Avi/Chegu Vinod had felt it is better to avoid vmexit itself, which
seems little complex to achieve currently.
OK, so the vmexit stays and we need to improve yield_to.
How about something like the below, that would allow breaking out of the
for-each-vcpu loop and simply going back into the vm, right?
---
kernel/sched/core.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b38f00e..5d5b355 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4272,7 +4272,10 @@ EXPORT_SYMBOL(yield);
* It's the caller's job to ensure that the target task struct
* can't go away on us before we can do any checks.
*
- * Returns true if we indeed boosted the target task.
+ * Returns:
+ * true (>0) if we indeed boosted the target task.
+ * false (0) if we failed to boost the target.
+ * -ESRCH if there's no task to yield to.
*/
bool __sched yield_to(struct task_struct *p, bool preempt)
{
@@ -4284,6 +4287,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
local_irq_save(flags);
rq = this_rq();
+ /*
+ * If we're the only runnable task on the rq, there's absolutely no
+ * point in yielding.
+ */
+ if (rq->nr_running == 1) {
+ yielded = -ESRCH;
+ goto out_irq;
+ }
+
again:
p_rq = task_rq(p);
double_rq_lock(rq, p_rq);
@@ -4293,13 +4305,13 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
}
if (!curr->sched_class->yield_to_task)
- goto out;
+ goto out_unlock;
if (curr->sched_class != p->sched_class)
- goto out;
+ goto out_unlock;
if (task_running(p_rq, p) || p->state)
- goto out;
+ goto out_unlock;
yielded = curr->sched_class->yield_to_task(rq, p, preempt);
if (yielded) {
@@ -4312,11 +4324,12 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
resched_task(p_rq->curr);
}
-out:
+out_unlock:
double_rq_unlock(rq, p_rq);
+out_irq:
local_irq_restore(flags);
- if (yielded)
+ if (yielded> 0)
schedule();
return yielded;
Yes, I think this is a nice idea. Any future users of yield_to
also would benefit from this. we will have to iterate only till first
attempt to yield_to.
I 'll run the test with this patch.
However Rik had a genuine concern in the cases where runqueue is not
equally distributed and lockholder might actually be on a different run
queue but not running.
Do you think instead of using rq->nr_running, we could get a global
sense of load using avenrun (something like avenrun/num_onlinecpus)
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