Re: [PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm

[Ingo Molnar]

* Rik van Riel <riel@xxxxxxxxxxx> wrote:

> On Mon, 2018-07-16 at 01:04 +0200, Ingo Molnar wrote:
> > * Rik van Riel <riel@xxxxxxxxxxx> wrote:
> > 
> > > +		/*
> > > +		 * Stop remote flushes for the previous mm.
> > > +		 * Skip the idle task; we never send init_mm TLB
> > > flushing IPIs,
> > > +		 * but the bitmap manipulation can cause cache
> > > line contention.
> > > +		 */
> > > +		if (real_prev != &init_mm) {
> > > +			VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu,
> > > +						mm_cpumask(real_pr
> > > ev)));
> > > +			cpumask_clear_cpu(cpu,
> > > mm_cpumask(real_prev));
> > 
> > BTW., could this optimization be (safely) extended to all (or most)
> > !task->mm 
> > kernel threads?
> > 
> > In particular softirq and threaded irq handlers could benefit greatly
> > I suspect in 
> > certain networking intense workloads that happen to active them.
> 
> Yes, it could.
> 
> Are there kernel threads that use something other than
> init_mm today?

Yeah, I think that's the typical case - so at minimum the comment should be fixed:

 > > > +	 * Skip the idle task; we never send init_mm TLB flushing IPIs,

and it should say 'skip kernel threads', right?

Thanks,

	Ingo