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Date: Thu, 14 Feb 2019 12:57:30 -0700
From: Khalid Aziz <khalid.aziz@...cle.com>
To: Dave Hansen <dave.hansen@...el.com>, juergh@...il.com, tycho@...ho.ws,
jsteckli@...zon.de, ak@...ux.intel.com, torvalds@...ux-foundation.org,
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mhocko@...e.com, catalin.marinas@....com, will.deacon@....com,
jmorris@...ei.org, konrad.wilk@...cle.com
Cc: deepa.srinivasan@...cle.com, chris.hyser@...cle.com, tyhicks@...onical.com,
dwmw@...zon.co.uk, andrew.cooper3@...rix.com, jcm@...hat.com,
boris.ostrovsky@...cle.com, kanth.ghatraju@...cle.com,
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pradeep.vincent@...cle.com, john.haxby@...cle.com, tglx@...utronix.de,
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linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v8 13/14] xpfo, mm: Defer TLB flushes for non-current
CPUs (x86 only)
On 2/14/19 10:42 AM, Dave Hansen wrote:
>> #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.
Of course. I will fix that.
>
>> + 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?
That sounds like a good idea. On the other hand, pending flush needs to
be kept track of entirely within arch/x86/mm/tlb.c and using a local
variable with scope limited to just that file feels like a lighter
weight implementation. I could go either way.
>
> 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.
>
>
That is a good point. It is not just wasteful, it is bound to have
performance impact even if slight.
>> +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().
>
I will fix that. Thanks!
--
Khalid
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