From: Avi Kivity <avi@redhat.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Nick Piggin <npiggin@suse.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux Memory Management List <linux-mm@kvack.org>,
the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast" flag
Date: Sat, 28 Mar 2009 12:54:19 +0300 [thread overview]
Message-ID: <49CDF3CB.8070000@redhat.com> (raw)
In-Reply-To: <49CDAF17.5060207@goop.org>
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.
WARNING: multiple messages have this Message-ID (diff)
From: Avi Kivity <avi@redhat.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Nick Piggin <npiggin@suse.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux Memory Management List <linux-mm@kvack.org>,
the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast" flag
Date: Sat, 28 Mar 2009 12:54:19 +0300 [thread overview]
Message-ID: <49CDF3CB.8070000@redhat.com> (raw)
In-Reply-To: <49CDAF17.5060207@goop.org>
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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next prev parent reply other threads:[~2009-03-28 9:54 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-03-27 20:31 [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast" flag Jeremy Fitzhardinge
2009-03-27 20:31 ` Jeremy Fitzhardinge
2009-03-28 3:48 ` Avi Kivity
2009-03-28 3:48 ` Avi Kivity
2009-03-28 5:01 ` Jeremy Fitzhardinge
2009-03-28 5:01 ` Jeremy Fitzhardinge
2009-03-28 7:54 ` Eric Dumazet
2009-03-28 7:54 ` Eric Dumazet
2009-03-28 12:31 ` Peter Zijlstra
2009-03-28 12:31 ` Peter Zijlstra
2009-03-28 9:54 ` Avi Kivity [this message]
2009-03-28 9:54 ` Avi Kivity
2009-03-28 12:27 ` Peter Zijlstra
2009-03-28 12:27 ` Peter Zijlstra
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