Re: [RFC 05/11] x86/mm: Rework lazy TLB mode and TLB freshness tracking

[Nadav Amit <nadav.amit@gmail.com>]

> On Jun 5, 2017, at 3:36 PM, Andy Lutomirski <luto@kernel.org> 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.
> 

[snip]

> @@ -67,133 +67,118 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
> {
> 

[snip]

> +		/* Resume remote flushes and then read tlb_gen. */
> +		cpumask_set_cpu(cpu, mm_cpumask(next));
> +		next_tlb_gen = atomic64_read(&next->context.tlb_gen);

It seems correct, but it got me somewhat confused at first.

Perhaps it worth a comment that a memory barrier is not needed since
cpumask_set_cpu() uses a locked-instruction. Otherwise, somebody may
even copy-paste it to another architecture...

Thanks,
Nadav
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