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Date: Thu, 14 Feb 2019 09:42:27 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Khalid Aziz <khalid.aziz@...cle.com>, juergh@...il.com, tycho@...ho.ws,
jsteckli@...zon.de, ak@...ux.intel.com, torvalds@...ux-foundation.org,
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Cc: deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
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Subject: Re: [RFC PATCH v8 13/14] xpfo, mm: Defer TLB flushes for non-current
CPUs (x86 only)
> #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().
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