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Message-ID: <49CDD7B4.4020701@cosmosbay.com>
Date: Sat, 28 Mar 2009 08:54:28 +0100
From: Eric Dumazet <dada1@...mosbay.com>
To: Jeremy Fitzhardinge <jeremy@...p.org>
CC: Avi Kivity <avi@...hat.com>, 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 a écrit :
> Avi Kivity wrote:
>> Jeremy Fitzhardinge wrote:
>>> get_user_pages_fast() relies on cross-cpu tlb flushes being a barrier
>>> between clearing and setting a pte, and before freeing a pagetable page.
>>> It usually does this by disabling interrupts to hold off IPIs, but
>>> some tlb flush implementations don't use IPIs for tlb flushes, and
>>> must use another mechanism.
>>>
>>> In this change, add in_gup_cpumask, which is a cpumask of cpus currently
>>> performing a get_user_pages_fast traversal of a pagetable. A cross-cpu
>>> tlb flush function can use this to determine whether it should hold-off
>>> on the flush until the gup_fast has finished.
>>>
>>> @@ -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.)
If I am not mistaken, shared futexes where hitting hard mm semaphore.
Then gup_fast was introduced in kernel/futex.c to remove this contention point.
Yet, this contention point was process specific, not a global one :)
And now, you want to add a global hot point, that would slow
down unrelated processes, only because they use shared futexes, thousand
times per second...
>
> Alternatively, it could have per-cpu flags and the other side could
> construct the mask (I originally had that, but this was simpler).
Simpler but would be a regression for legacy applications still using shared
futexes (because statically linked with old libc)
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