Re: [RFC][PATCH] Improving directed yield scalability for PLEhandler

[Andrew Theurer]

On Tue, 2012-09-11 at 11:38 +0530, Raghavendra K T wrote:
> On 09/11/2012 01:42 AM, Andrew Theurer wrote:
> > On Mon, 2012-09-10 at 19:12 +0200, Peter Zijlstra wrote:
> >> On Mon, 2012-09-10 at 22:26 +0530, Srikar Dronamraju wrote:
> >>>> +static bool __yield_to_candidate(struct task_struct *curr, struct task_struct *p)
> >>>> +{
> >>>> +     if (!curr->sched_class->yield_to_task)
> >>>> +             return false;
> >>>> +
> >>>> +     if (curr->sched_class != p->sched_class)
> >>>> +             return false;
> >>>
> >>>
> >>> Peter,
> >>>
> >>> Should we also add a check if the runq has a skip buddy (as pointed out
> >>> by Raghu) and return if the skip buddy is already set.
> >>
> >> Oh right, I missed that suggestion.. the performance improvement went
> >> from 81% to 139% using this, right?
> >>
> >> It might make more sense to keep that separate, outside of this
> >> function, since its not a strict prerequisite.
> >>
> >>>>
> >>>> +     if (task_running(p_rq, p) || p->state)
> >>>> +             return false;
> >>>> +
> >>>> +     return true;
> >>>> +}
> >>
> >>
> >>>> @@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p,
> >>> bool preempt)
> >>>>        rq = this_rq();
> >>>>
> >>>>   again:
> >>>> +     /* optimistic test to avoid taking locks */
> >>>> +     if (!__yield_to_candidate(curr, p))
> >>>> +             goto out_irq;
> >>>> +
> >>
> >> So add something like:
> >>
> >> 	/* Optimistic, if we 'raced' with another yield_to(), don't bother */
> >> 	if (p_rq->cfs_rq->skip)
> >> 		goto out_irq;
> >>>
> >>>
> >>>>        p_rq = task_rq(p);
> >>>>        double_rq_lock(rq, p_rq);
> >>>
> >>>
> >> But I do have a question on this optimization though,.. Why do we check
> >> p_rq->cfs_rq->skip and not rq->cfs_rq->skip ?
> >>
> >> That is, I'd like to see this thing explained a little better.
> >>
> >> Does it go something like: p_rq is the runqueue of the task we'd like to
> >> yield to, rq is our own, they might be the same. If we have a ->skip,
> >> there's nothing we can do about it, OTOH p_rq having a ->skip and
> >> failing the yield_to() simply means us picking the next VCPU thread,
> >> which might be running on an entirely different cpu (rq) and could
> >> succeed?
> >
> > Here's two new versions, both include a __yield_to_candidate(): "v3"
> > uses the check for p_rq->curr in guest mode, and "v4" uses the cfs_rq
> > skip check.  Raghu, I am not sure if this is exactly what you want
> > implemented in v4.
> >
> 
> Andrew, Yes that is what I had. I think there was a mis-understanding. 
> My intention was to if there is a directed_yield happened in runqueue 
> (say rqA), do not bother to directed yield to that. But unfortunately as 
> PeterZ pointed that would have resulted in setting next buddy of a 
> different run queue than rqA.
> So we can drop this "skip" idea. Pondering more over what to do? can we 
> use next buddy itself ... thinking..

As I mentioned earlier today, I did not have your changes from kvm.git
tree when I tested my changes.  Here are your changes and my changes
compared:

			  throughput in MB/sec

kvm_vcpu_on_spin changes:  4636 +/- 15.74%
yield_to changes:	   4515 +/- 12.73%

I would be inclined to stick with your changes which are kept in kvm
code.  I did try both combined, and did not get good results:

both changes:		   4074 +/- 19.12%

So, having both is probably not a good idea.  However, I feel like
there's more work to be done.  With no over-commit (10 VMs), total
throughput is 23427 +/- 2.76%.  A 2x over-commit will no doubt have some
overhead, but a reduction to ~4500 is still terrible.  By contrast,
8-way VMs with 2x over-commit have a total throughput roughly 10% less
than 8-way VMs with no overcommit (20 vs 10 8-way VMs on 80 cpu-thread
host).  We still have what appears to be scalability problems, but now
it's not so much in runqueue locks for yield_to(), but now
get_pid_task():

perf on host:

32.10% 320131 qemu-system-x86 [kernel.kallsyms] [k] get_pid_task
11.60% 115686 qemu-system-x86 [kernel.kallsyms] [k] _raw_spin_lock
10.28% 102522 qemu-system-x86 [kernel.kallsyms] [k] yield_to
 9.17%  91507 qemu-system-x86 [kvm]             [k] kvm_vcpu_on_spin
 7.74%  77257 qemu-system-x86 [kvm]             [k] kvm_vcpu_yield_to
 3.56%  35476 qemu-system-x86 [kernel.kallsyms] [k] __srcu_read_lock
 3.00%  29951 qemu-system-x86 [kvm]             [k] __vcpu_run
 2.93%  29268 qemu-system-x86 [kvm_intel]       [k] vmx_vcpu_run
 2.88%  28783 qemu-system-x86 [kvm]             [k] vcpu_enter_guest
 2.59%  25827 qemu-system-x86 [kernel.kallsyms] [k] __schedule
 1.40%  13976 qemu-system-x86 [kernel.kallsyms] [k] _raw_spin_lock_irq
 1.28%  12823 qemu-system-x86 [kernel.kallsyms] [k] resched_task
 1.14%  11376 qemu-system-x86 [kvm_intel]       [k] vmcs_writel
 0.85%   8502 qemu-system-x86 [kernel.kallsyms] [k] pick_next_task_fair
 0.53%   5315 qemu-system-x86 [kernel.kallsyms] [k] native_write_msr_safe
 0.46%   4553 qemu-system-x86 [kernel.kallsyms] [k] native_load_tr_desc

get_pid_task() uses some rcu fucntions, wondering how scalable this
is....  I tend to think of rcu as -not- having issues like this... is
there a rcu stat/tracing tool which would help identify potential
problems?

-Andrew

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/