Re: [PATCH 2/6] x86: mm: rip out complicated, out-of-date, buggy TLB flushing

From: Rik van Riel
Date: Tue Apr 22 2014 - 12:55:09 EST


On 04/21/2014 02:24 PM, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> I think the flush_tlb_mm_range() code that tries to tune the
> flush sizes based on the CPU needs to get ripped out for
> several reasons:
>
> 1. It is obviously buggy. It uses mm->total_vm to judge the
> task's footprint in the TLB. It should certainly be using
> some measure of RSS, *NOT* ->total_vm since only resident
> memory can populate the TLB.
> 2. Haswell, and several other CPUs are missing from the
> intel_tlb_flushall_shift_set() function. Thus, it has been
> demonstrated to bitrot quickly in practice.
> 3. It is plain wrong in my vm:
> [ 0.037444] Last level iTLB entries: 4KB 0, 2MB 0, 4MB 0
> [ 0.037444] Last level dTLB entries: 4KB 0, 2MB 0, 4MB 0
> [ 0.037444] tlb_flushall_shift: 6
> Which leads to it to never use invlpg.
> 4. The assumptions about TLB refill costs are wrong:
> http://lkml.kernel.org/r/1337782555-8088-3-git-send-email-alex.shi@xxxxxxxxx
> (more on this in later patches)
> 5. I can not reproduce the original data: https://lkml.org/lkml/2012/5/17/59
> I believe the sample times were too short. Running the
> benchmark in a loop yields times that vary quite a bit.
>
> Note that this leaves us with a static ceiling of 1 page. This
> is a conservative, dumb setting, and will be revised in a later
> patch.
>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Acked-by: Rik van Riel <riel@xxxxxxxxxx>


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