linux-kernel - Re: [RFC PATCH v2 3/3] x86/mm/tlb: Avoid deferring PTI flushes on shootdown

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Date:   Tue, 27 Aug 2019 23:57:27 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH v2 3/3] x86/mm/tlb: Avoid deferring PTI flushes on
 shootdown

> On Aug 27, 2019, at 4:07 PM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> On Fri, Aug 23, 2019 at 11:13 PM Nadav Amit <namit@...are.com> wrote:
>> When a shootdown is initiated, the initiating CPU has cycles to burn as
>> it waits for the responding CPUs to receive the IPI and acknowledge it.
>> In these cycles it is better to flush the user page-tables using
>> INVPCID, instead of deferring the TLB flush.
>> 
>> The best way to figure out whether there are cycles to burn is arguably
>> to expose from the SMP layer when an acknowledgment is received.
>> However, this would break some abstractions.
>> 
>> Instead, use a simpler solution: the initiating CPU of a TLB shootdown
>> would not defer PTI flushes. It is not always a win, relatively to
>> deferring user page-table flushes, but it prevents performance
>> regression.
>> 
>> Signed-off-by: Nadav Amit <namit@...are.com>
>> ---
>> arch/x86/include/asm/tlbflush.h |  1 +
>> arch/x86/mm/tlb.c               | 10 +++++++++-
>> 2 files changed, 10 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
>> index da56aa3ccd07..066b3804f876 100644
>> --- a/arch/x86/include/asm/tlbflush.h
>> +++ b/arch/x86/include/asm/tlbflush.h
>> @@ -573,6 +573,7 @@ struct flush_tlb_info {
>>        unsigned int            initiating_cpu;
>>        u8                      stride_shift;
>>        u8                      freed_tables;
>> +       u8                      shootdown;
> 
> I find the name "shootdown" to be confusing.  How about "more_than_one_cpu”?

I think the current semantic is more of “includes remote cpus”. How about
calling it “local_only”, and negating its value?