linux-kernel - Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages

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Message-ID: <49CDF3CB.8070000@redhat.com>
Date:	Sat, 28 Mar 2009 12:54:19 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast"
 flag

Jeremy Fitzhardinge wrote:
>>> @@ -255,6 +260,10 @@ int get_user_pages_fast(unsigned long start, 
>>> int nr_pages, int write,
>>>      * address down to the the page and take a ref on it.
>>>      */
>>>     local_irq_disable();
>>> +
>>> +    cpu = smp_processor_id();
>>> +    cpumask_set_cpu(cpu, in_gup_cpumask);
>>> +
>>
>> This will bounce a cacheline, every time.  Please wrap in CONFIG_XEN 
>> and skip at runtime if Xen is not enabled.
>
> Every time?  Only when running successive gup_fasts on different cpus, 
> and only twice per gup_fast. (What's the typical page count?  I see 
> that kvm and lguest are page-at-a-time users, but presumably direct IO 
> has larger batches.)

Databases will often issue I/Os of 1 or 2 pages.  But not regressing kvm 
should be sufficient motivation.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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