linux-kernel - Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

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Message-ID: <51ABF410.4090109@linux.vnet.ibm.com>
Date:	Mon, 03 Jun 2013 07:10:32 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Jiannan Ouyang <ouyang@...pitt.edu>, Gleb Natapov <gleb@...hat.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, x86@...nel.org,
	konrad.wilk@...cle.com, "H. Peter Anvin" <hpa@...or.com>,
	pbonzini@...hat.com, linux-doc@...r.kernel.org,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	xen-devel@...ts.xensource.com,
	Peter Zijlstra <peterz@...radead.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	stefano.stabellini@...citrix.com, andi@...stfloor.org,
	attilio.rao@...rix.com, gregkh@...e.de, agraf@...e.de,
	chegu vinod <chegu_vinod@...com>,
	torvalds@...ux-foundation.org, Avi Kivity <avi.kivity@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	KVM <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
	stephan.diestelhorst@....com, Rik van Riel <riel@...hat.com>,
	Andrew Jones <drjones@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

On 06/02/2013 09:50 PM, Jiannan Ouyang wrote:
> On Sun, Jun 2, 2013 at 1:07 AM, Gleb Natapov <gleb@...hat.com> wrote:
>
>> High level question here. We have a big hope for "Preemptable Ticket
>> Spinlock" patch series by Jiannan Ouyang to solve most, if not all,
>> ticketing spinlocks in overcommit scenarios problem without need for PV.
>> So how this patch series compares with his patches on PLE enabled processors?
>>
>
> No experiment results yet.
>
> An error is reported on a 20 core VM. I'm during an internship
> relocation, and will start work on it next week.

Preemptable spinlocks' testing update:
I hit the same softlockup problem while testing on 32 core machine with
32 guest vcpus that Andrew had reported.

After that i started tuning TIMEOUT_UNIT, and when I went till (1<<8),
things seemed to be manageable for undercommit cases.
But I still see degradation for undercommit w.r.t baseline itself on 32
core machine (after tuning).

(37.5% degradation w.r.t base line).
I can give the full report after the all tests complete.

For over-commit cases, I again started hitting softlockups (and
degradation is worse). But as I said in the preemptable thread, the
concept of preemptable locks looks promising (though I am still not a
fan of  embedded TIMEOUT mechanism)

Here is my opinion of TODOs for preemptable locks to make it better ( I
think I need to paste in the preemptable thread also)

1. Current TIMEOUT UNIT seem to be on higher side and also it does not
scale well with large guests and also overcommit. we need to have a
sort of adaptive mechanism and better is sort of different TIMEOUT_UNITS
for different types of lock too. The hashing mechanism that was used in
Rik's spinlock backoff series fits better probably.

2. I do not think TIMEOUT_UNIT itself would work great when we have a
big queue (for large guests / overcommits) for lock.
one way is to add a PV hook that does yield hypercall immediately for
the waiters above some THRESHOLD so that they don't burn the CPU.
( I can do POC to check if  that idea works in improving situation
at some later point of time)

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