linux-kernel - Re: [PATCH v3] sched/fair: Add advisory flag for borrowing a timeslice

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Message-ID: <54749F77.50905@redhat.com>
Date:	Tue, 25 Nov 2014 10:25:43 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Khalid Aziz <khalid.aziz@...cle.com>, tglx@...utronix.de,
	corbet@....net, mingo@...hat.com, hpa@...or.com,
	peterz@...radead.org, akpm@...ux-foundation.org,
	rientjes@...gle.com, ak@...ux.intel.com, mgorman@...e.de,
	liwanp@...ux.vnet.ibm.com, raistlin@...ux.it,
	kirill.shutemov@...ux.intel.com, atomlin@...hat.com,
	avagin@...nvz.org, gorcunov@...nvz.org, serge.hallyn@...onical.com,
	athorlton@....com, oleg@...hat.com, vdavydov@...allels.com,
	daeseok.youn@...il.com, keescook@...omium.org,
	yangds.fnst@...fujitsu.com, sbauer@....utah.edu,
	vishnu.ps@...sung.com, axboe@...com, paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v3] sched/fair: Add advisory flag for borrowing a timeslice

On 11/25/2014 09:52 AM, Khalid Aziz wrote:
> On 11/24/2014 07:03 PM, Rik van Riel wrote:
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>>
>> On 11/24/2014 03:56 PM, Khalid Aziz wrote:
>>> sched/fair: Add advisory flag for borrowing a timeslice
>>>
>>> This patch adds a way for a task to request to borrow one
>>> timeslice from future if it is about to be preempted, so it could
>>> delay preemption and complete any critical task it is in the middle
>>> of.
>>>
>>> This feature helps with performance on databases and has been used
>>> for many years on other OSs by the databases. This feature helps in
>>> situation where a task acquires a lock before performing a critical
>>> operation on the database and happens to get preempted
>>
>> Why don't the other tasks that want the lock sleep on the
>> lock?
>>
>> I can see this "solution" help mostly with userspace spinlocks,
>> which are relics of a past era that need to die. There is no
>> way userspace spinlocks will not fail miserably on virtual
>> machines, and it is time to get rid of them.
> 
> This solution indeed is for userspace spinlocks. Database code has been
> written with all critical locking implemented in userspace (as I have
> been told by database folks. I am not a database guy).

They should fix that.

The scheme you propose can really only work on bare metal,
and not on virtual machines. That improves the problem for,
what, 60% of new installs (and dropping)?
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