Re: [RFC PATCH v8 13/14] xpfo, mm: Defer TLB flushes for non-current CPUs (x86 only)

[Dave Hansen]

>  #endif
> +
> +	/* If there is a pending TLB flush for this CPU due to XPFO
> +	 * flush, do it now.
> +	 */

Don't forget CodingStyle in all this, please.

> +	if (cpumask_test_and_clear_cpu(cpu, &pending_xpfo_flush)) {
> +		count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED);
> +		__flush_tlb_all();
> +	}

This seems to exist in parallel with all of the cpu_tlbstate
infrastructure.  Shouldn't it go in there?

Also, if we're doing full flushes like this, it seems a bit wasteful to
then go and do later things like invalidate_user_asid() when we *know*
that the asid would have been flushed by this operation.  I'm pretty
sure this isn't the only __flush_tlb_all() callsite that does this, so
it's not really criticism of this patch specifically.  It's more of a
structural issue.


> +void xpfo_flush_tlb_kernel_range(unsigned long start, unsigned long end)
> +{

This is a bit lightly commented.  Please give this some good
descriptions about the logic behind the implementation and the tradeoffs
that are in play.

This is doing a local flush, but deferring the flushes on all other
processors, right?  Can you explain the logic behind that in a comment
here, please?  This also has to be called with preemption disabled, right?

> +	struct cpumask tmp_mask;
> +
> +	/* Balance as user space task's flush, a bit conservative */
> +	if (end == TLB_FLUSH_ALL ||
> +	    (end - start) > tlb_single_page_flush_ceiling << PAGE_SHIFT) {
> +		do_flush_tlb_all(NULL);
> +	} else {
> +		struct flush_tlb_info info;
> +
> +		info.start = start;
> +		info.end = end;
> +		do_kernel_range_flush(&info);
> +	}
> +	cpumask_setall(&tmp_mask);
> +	cpumask_clear_cpu(smp_processor_id(), &tmp_mask);
> +	cpumask_or(&pending_xpfo_flush, &pending_xpfo_flush, &tmp_mask);
> +}

Fun.  cpumask_setall() is non-atomic while cpumask_clear_cpu() and
cpumask_or() *are* atomic.  The cpumask_clear_cpu() is operating on
thread-local storage and doesn't need to be atomic.  Please make it
__cpumask_clear_cpu().