Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast"flag

[Eric Dumazet]

Jeremy Fitzhardinge a Ãcrit :
> 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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