Re: [RFC][PATCH] Improving directed yield scalability for PLE handler

From: Raghavendra K T
Date: Mon Sep 10 2012 - 10:46:27 EST

On 09/08/2012 01:12 AM, Andrew Theurer wrote:
On Fri, 2012-09-07 at 23:36 +0530, Raghavendra K T wrote:
CCing PeterZ also.

On 09/07/2012 06:41 PM, Andrew Theurer wrote:
I have noticed recently that PLE/yield_to() is still not that scalable
for really large guests, sometimes even with no CPU over-commit. I have
a small change that make a very big difference.
We are indeed avoiding CPUS in guest mode when we check
task->flags& PF_VCPU in vcpu_on_spin path. Doesn't that suffice?
My understanding is that it checks if the candidate vcpu task is in
guest mode (let's call this vcpu g1vcpuN), and that vcpu will not be a
target to yield to if it is already in guest mode. I am concerned about
a different vcpu, possibly from a different VM (let's call it g2vcpuN),
but it also located on the same runqueue as g1vcpuN -and- running. That
vcpu, g2vcpuN, may also be doing a directed yield, and it may already be
holding the rq lock. Or it could be in guest mode. If it is in guest
mode, then let's still target this rq, and try to yield to g1vcpuN.
However, if g2vcpuN is not in guest mode, then don't bother trying.

- If a non vcpu task was currently running, this change can ignore request to yield to a target vcpu. The target vcpu could be the most eligible vcpu causing other vcpus to do ple exits.
Is it possible to modify the check to deal with only vcpu tasks?

- Should we use p_rq->cfs_rq->skip instead to let us know that some yield was active at this time?


Cpu 1 cpu2 cpu3
a1 a2 a3
b1 b2 b3
c2(yield target of a1) c3(yield target of a2)

If vcpu a1 is doing directed yield to vcpu c2; current vcpu a2 on target cpu is also doing a directed yield(to some vcpu c3). Then this change will only allow vcpu a2 will do a schedule() to b2 (if a2 -> c3 yield is successful). Do we miss yielding to a vcpu c2?
a1 might not find a suitable vcpu to yield and might go back to spinning. Is my understanding correct?

Patch include below.

Here's the new, v2 result with the previous two:

10 VMs, 16-way each, all running dbench (2x cpu over-commit)
throughput +/- stddev
----- -----
ple on: 2552 +/- .70%
ple on: w/fixv1: 4621 +/- 2.12% (81% improvement)
ple on: w/fixv2: 6115* (139% improvement)

The numbers look great.

[*] I do not have stdev yet because all 10 runs are not complete

for v1 to v2, host CPU dropped from 60% to 50%. Time in spin_lock() is
also dropping:


So this seems to be working. However I wonder just how far we can take
this. Ideally we need to be in<3-4% in host for PLE work, like I
observe for the 8-way VMs. We are still way off.


signed-off-by: Andrew Theurer<habanero@xxxxxxxxxxxxxxxxxx>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fbf1fd0..c767915 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4844,6 +4844,9 @@ bool __sched yield_to(struct task_struct *p, bool

p_rq = task_rq(p);
+ if (task_running(p_rq, p) || p->state || !(p_rq->curr->flags&

While we are checking the flags of p_rq->curr task, the task p can migrate to some other runqueue. In this case will we miss yielding to the most eligible vcpu?

+ goto out_no_unlock;
+ }

We dont need parenthesis above.

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