Re: [PATCH v2 05/10] x86/mm: Rework lazy TLB mode and TLB freshness tracking

[Andy Lutomirski]

On Tue, Jun 13, 2017 at 11:09 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
> On 14/06/17 06:56, Andy Lutomirski wrote:
>> x86's lazy TLB mode used to be fairly weak -- it would switch to
>> init_mm the first time it tried to flush a lazy TLB.  This meant an
>> unnecessary CR3 write and, if the flush was remote, an unnecessary
>> IPI.
>>
>> Rewrite it entirely.  When we enter lazy mode, we simply remove the
>> cpu from mm_cpumask.  This means that we need a way to figure out
>> whether we've missed a flush when we switch back out of lazy mode.
>> I use the tlb_gen machinery to track whether a context is up to
>> date.
>>
>> Note to reviewers: this patch, my itself, looks a bit odd.  I'm
>> using an array of length 1 containing (ctx_id, tlb_gen) rather than
>> just storing tlb_gen, and making it at array isn't necessary yet.
>> I'm doing this because the next few patches add PCID support, and,
>> with PCID, we need ctx_id, and the array will end up with a length
>> greater than 1.  Making it an array now means that there will be
>> less churn and therefore less stress on your eyeballs.
>>
>> NB: This is dubious but, AFAICT, still correct on Xen and UV.
>> xen_exit_mmap() uses mm_cpumask() for nefarious purposes and this
>> patch changes the way that mm_cpumask() works.  This should be okay,
>> since Xen *also* iterates all online CPUs to find all the CPUs it
>> needs to twiddle.
>
> There is a allocation failure path in xen_drop_mm_ref() which might
> be wrong with this patch. As this path should be taken only very
> unlikely I'd suggest to remove the test for mm_cpumask() bit zero in
> this path.
>

Right, fixed.

>
> Juergen