Re: [PATCH] x86,tlb: update mm_cpumask lazily

[Rik van Riel]

On Fri, 2024-11-08 at 12:03 -0800, Dave Hansen wrote:
> On 11/8/24 11:31, Rik van Riel wrote:
> >  		/* Start receiving IPIs and then read tlb_gen (and
> > LAM below) */
> > -		if (next != &init_mm)
> > +		if (next != &init_mm && !cpumask_test_cpu(cpu,
> > mm_cpumask(next)))
> >  			cpumask_set_cpu(cpu, mm_cpumask(next));
> >  		next_tlb_gen = atomic64_read(&next-
> > >context.tlb_gen);
> 
> If we're worried about contention on mm_cpumask(), then this hunk
> makes
> sense independently of the lazy updating. We might want to take this
> hunk forward before we do the rest because this seems like a no-
> brainer.
> 
If we always clear the CPU in the mm_cpumask when prev != next,
wouldn't that result in that CPU's bit being clear (and needing
to be set) for next when prev != next?

What am I missing?

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